moving helper/schema into the ineternal/legacy tree

If an attribute was not wholly known, helper/schema was skipping the
`validateType` function which (among other things) returned deprecation
messages. This PR checks for deprecation before returning when skipping
validateType.

Nil values were not previously expected during validation, but they can
appear in some situations with the new protocol. Add checks to prevent
using zero reflect.Values.

If there are unknowns, the block may have come from a dynamic
declaration, and we can't validate MinItems. Once the blocks are
expanded, we will get the full config for validation without any unknown
values.

While there was already a check in the sdk to filter unknowns from
validation, it missed the case where those were in simple lists and maps.

This experiment is no longer needed for handling computed blocks, since
the legacy SDK can't reasonably handle Dynamic types, we need to remove
this before the final release.

Remove LegacySchema functions as well, since handling SkipCoreTypeCheck
was the only thing left they were handling.

The new type system only has a Number type, but helper schema
differentiates between Int and Float values. Verify that a new config
value is an integer during Validate, because the existing WeakDecode
validation will decode a float value into an integer while the config
FieldReader will attempt to parse the float exactly.

Since we're limiting this to protoV5, we can be certain that any valid
config value will be converted to an `int` type by the shims. The only
case where an integral float value will appear is if the integer is out
of range for the systems `int` type, but we also need to prevent that
anyway since it would fail to read in the same manner.

This reverts commit 4c0c74571de9c96ad2902ccf4af962ec495cd5d4.

When running in v0.12-and-higher mode, this will cause the SDK to report
the type of the attribute as "any", effectively skipping type checking
on the Core side altogether and checking only in the SDK and provider
code.

The practical impact of this is to restore the v0.11-style checking
behavior of allowing object values to be missing certain attributes as
long as they are marked as optional in the schema. The SDK can do this
because it uses a unified schema model for both object values and nested
blocks, while Terraform Core only supports the idea of "optional" when
talking about attributes in nested blocks.

This is a continuation of the pile of workarounds that also includes
the ConfigMode and AsSingle fields, allowing providers to selectively opt
out of new v0.12 behaviors in situations where they conflict with
decisions made in the design of the providers in our old world where
Terraform Core delegated _all_ validation to providers.

This is designed as an opt-in so that we can limit its impact only to
specific cases where it's needed and minimize the risk of regressions
elsewhere. Providers should use this sparingly only in situations where
prevailing usage disagrees with the new expectations of Terraform Core in
v0.12.

This commit only adds the flag, and does not implement any behavior for it
yet. That means this commit can exist in both the v0.11 and v0.12
codebases, allowing for API compatibility. A subsequent commit for v0.12
(not included in v0.11) will then implement this behavior.

This setting indicates that an attribute defined as TypeList or TypeSet
should be presented to Terraform Core as a single value instead when
running in Terraform v0.12 or later. It has no effect for Terraform v0.10
or v0.11.

This commit just introduces the setting without any associated behavior,
so it can be included in both the v0.12 and v0.11 branches. A subsequent
commit only to the v0.12 branch will introduce the behavior as part of
the protocol version 5 shims.

This allows a provider developer slightly more control over how an SDK
schema is mapped into the Terraform configuration language, overriding
some default assumptions.

ConfigMode overrides the default assumption that a schema with
an Elem of type *Resource is to be mapped to configuration as a nested
block, allowing mapping as an attribute containing an object type instead.

These behaviors only apply when a provider is being used with Terraform
v0.12 or later. They are ignored altogether in Terraform v0.11 mode, to
preserve compatibility. We are adding these primarily to allow the v0.12
version of a resource type schema to be specified to match the prevailing
usage of it in existing configurations, in situations where the default
mapping to v0.12 concepts is not appropriate.

This commit adds only the fields themselves and the InternalValidate rules
for them. A subsequent commit for Terraform v0.12 will add the behavior
as part of the protocol version 5 shim layer.

This comment seems to imply that you can put CRUD functions on nested schema.Resource objects.

The comment goes back to the first commit to this file, but AFAICT this functionality has never been implemented.

Add feature flag to allow special proto 5 behavior in helper/schema.
This is Meant to be used as a last resort for shim-related bugs.

Sets rely on diffs being complete for all elements, even when they are
unchanged. When encountering a DiffSuppressFunc inside a set the diffs
were being dropped entirely, possible causing set elements to be lost.

With the introduction of explicit "null" in 0.12 it's possible for a value
that is unknown during plan to become a known null during apply, so we
need to slightly weaken our validation rules to accommodate that, in
particular skipping the validation of conflicting attributes if the result
could potentially be valid after the unknown values become known.

This change is in the codepath that is common to both 0.12 and 0.11
callers, but that's safe because 0.11 re-runs validation during the apply
step and so will still catch problems here, albeit in the apply step
rather than in the plan step, thus matching the 0.12 behavior. This new
behavior is a superset of the old in the sense that everything that was
valid before is still valid.

The implementation here also causes us to skip all other validation for
an attribute whose value is unknown. Most of the downstream validation
functions handle this directly anyway, but again this doesn't add any new
failure cases, and should clean up some of the rough edges we've seen with
unknown values in 0.11 once people upgrade to 0.12-compatible providers.
Any issues we now short-circuit during planning will still be caught
during apply.

While working on this I found that the existing "Not a list" test was not
actually testing the correct behavior, so this also includes a tweak to
that to ensure that it really is checking the "should be a list" path
rather than the "cannot be set" codepath it was inadvertently testing
before.

When the following conditions were met:
* Schema attribute with a primative type (e.g. Type: TypeString) and Computed: true
* Old state of attribute set to zero value for type (e.g. "")
* Old state ID of resource set to non-empty (e.g. existing resource)

Attempting to use CustomizeDiff with SetNewComputed() would result in the difference  previously being discarded. This update ensures that previous zero values or resource existence does not influence the propagation of the computed update.

Previously:

```
--- FAIL: TestSetNewComputed (0.00s)
    --- FAIL: TestSetNewComputed/NewComputed_should_always_propagate (0.00s)
        resource_diff_test.go:684: Expected (*terraform.InstanceDiff)(0xc00051cea0)({
             mu: (sync.Mutex) {
              state: (int32) 0,
              sema: (uint32) 0
             },
             Attributes: (map[string]*terraform.ResourceAttrDiff) (len=1) {
              (string) (len=3) "foo": (*terraform.ResourceAttrDiff)(0xc0003dcec0)({
               Old: (string) "",
               New: (string) "",
               NewComputed: (bool) true,
               NewRemoved: (bool) false,
               NewExtra: (interface {}) <nil>,
               RequiresNew: (bool) false,
               Sensitive: (bool) false,
               Type: (terraform.DiffAttrType) 0
              })
             },
             Destroy: (bool) false,
             DestroyDeposed: (bool) false,
             DestroyTainted: (bool) false,
             Meta: (map[string]interface {}) <nil>
            })
            , got (*terraform.InstanceDiff)(0xc00051ce80)({
             mu: (sync.Mutex) {
              state: (int32) 0,
              sema: (uint32) 0
             },
             Attributes: (map[string]*terraform.ResourceAttrDiff) {
             },
             Destroy: (bool) false,
             DestroyDeposed: (bool) false,
             DestroyTainted: (bool) false,
             Meta: (map[string]interface {}) <nil>
            })

--- FAIL: TestSchemaMap_Diff (0.01s)
    --- FAIL: TestSchemaMap_Diff/79-NewComputed_should_always_propagate_with_CustomizeDiff (0.00s)
        schema_test.go:3289: expected:
            *terraform.InstanceDiff{mu:sync.Mutex{state:0, sema:0x0}, Attributes:map[string]*terraform.ResourceAttrDiff{"foo":*terraform.ResourceAttrDiff{Old:"", New:"", NewComputed:true, NewRemoved:false, NewExtra:interface {}(nil), RequiresNew:false, Sensitive:false, Type:0x0}}, Destroy:false, DestroyDeposed:false, DestroyTainted:false, Meta:map[string]interface {}(nil)}

            got:
            <nil>

FAIL
FAIL  github.com/hashicorp/terraform/helper/schema  0.825s
```

This is no longer effective and should not be used in any new schema.

Terraform now handles any actual "diffing" of resource, and the existing
Diff functions are only used to shim the schema.Provider to the new
methods. Since terraform is handling what used to be the Diff, the
provider now should not modify the diff based on RequiresNew due to it
interfering with the ignore_changes handling.

The helper resource tests won't pass for now, as they use a
terraform.MockProvider which can't be used in the schema.Provider shims.

This exposes the internal schemaMap for use by the new provider shims.

Signed-off-by: Guillaume J. Charmes <gcharmes@magicleap.com>

For historical reasons, the handling of element types for maps is inconsistent with other collection types.

Here we begin a multi-step process to make it consistent, starting by supporting both the "consistent" form of using a schema.Schema and an existing erroneous form of using a schema.Type directly. In subsequent commits we will phase out the erroneous form and require the schema.Schema approach, the same as we do for TypeList and TypeSet.

slight change to go fmt coming in 0.10

This new codepath with the getDiff "customzed" return value, along with
the associated test need to be removed as soon as we can support unset
fields from the config, so we don't continue to carry this broken
behavior forward any longer than needed.

This extends the internal diffChange method so that ResourceDiff's
implementation of it can report back whether or not the value came from
a customized diff.

This is an effort to work to preserve the pre-ResourceDiff behaviour
that ignores the diff for computed keys when the old value was populated
but the new value wasn't - this behaviour is actually being depended on
by users that are using it to exploit using zero values in modules. This
should allow both scenarios to co-exist by shifting the NewComputed
exemption over to exempting values that come from diff customization.