command/format: Fix rendering of unknown elements in set/map/list

In Terraform 0.11 and earlier we just silently ignored undeclared
variables in -var-file and the automatically-loaded .tfvars files. This
was a bad user experience for anyone who made a typo in a variable name
and got no feedback about it, so we made this an error for 0.12.

However, several users are now relying on the silent-ignore behavior for
automation scenarios where they pass the same .tfvars file to all
configurations in their organization and expect Terraform to ignore any
settings that are not relevant to a specific configuration. We never
intentionally supported that, but we don't want to immediately break that
workflow during 0.12 upgrade.

As a compromise, then, we'll make this a warning for v0.12.0 that contains
a deprecation notice suggesting to move to using environment variables
for this "cross-configuration variables" use-case. We don't produce errors
for undeclared variables in environment variables, even though that
potentially causes the same UX annoyance as ignoring them in vars files,
because environment variables are assumed to live in the user's session
and this it would be very inconvenient to have to unset such variables
when moving between directories. Their "ambientness" makes them a better
fit for these automatically-assigned general variable values that may or
may not be used by a particular configuration.

This can revert to being an error in a future major release, after users
have had the opportunity to migrate their automation solutions over to
use environment variables.

We don't seem to have any tests covering this specific situation right
now. That isn't ideal, but this change is so straightforward that it would
be relatively expensive to build new targeted test cases for it and so
I instead just hand-tested that it is indeed now producing a warning where
we were previously producing an error. Hopefully if there is any more
substantial work done on this codepath in future that will be our prompt
to add some unit tests for this.

In an ideal world, providers are supposed to respond to errors during
apply by returning a partial new state alongside the error diagnostics.
In practice though, our SDK leaves the new value set to nil for certain
errors, which was causing Terraform to "forget" the object altogether by
assuming that the provider intended to say "null".

We now adjust that assumption to apply only in the delete case. In all
other cases (including updates) we retain the prior state if the new
state is given as nil. Although we could potentially fix this in the SDK
itself, I expect this is a likely bug in other future SDKs for other
languages too, so this new assumption is a safer one to make to be
resilient to data loss when providers don't behave perfectly.

Providers that return both nil new value and no errors are considered
buggy, but unfortunately that applies to the mocks in many of our tests,
so for pragmatic reasons we can't generate an error for that case as we do
for other "should never happen" situations. Instead, we'll just retain the
prior value in the state so the user can retry.

backend/remote: fix a small typo

We missed this on the initial update pass because this was calling
directly into the module package API rather than going through the Meta
methods that we updated for the new config loader.

m.installModules here is the same method that "terraform init" is using
for this purpose, ensuring the two will behave the same way. This changes
the output a little compared to the old installer, but it still includes
the important information about where each module is coming from.

Some fixes for the provider shims

We need these changes in master for testing, worry about these test
after.

Check if there wasn't any real diff attributes first, before returning
the original state in PlanResourceChange.

This adds unexpected values in some cases, and since the case this
handles is only within set objects, we'll deal woth this when tackling
the sets themselves.

Plan can change known values too, which we can't match in sets. We'll
find another way to normalize these eithout losing plan values.

Don't lose default values set within a nested block.

This possibility was lost in the rewrite to use HCL2, but it's used by
a number of external utilities and text editor integrations, so we'll
restore it here.

Using the stdin/stdout mode is generally preferable for text editor use
since it allows formatting of the in-memory buffer rather than directly
the file on disk, but for editors that don't have support for that sort of
tooling it can be convenient to just launch a single command and directly
modify the on-disk file.

Since the HCL formatter only works with tokens, it can in principle be
called with any input and produce some output. However, when given invalid
syntax it will tend to produce nonsensical results that may drastically
change the input file and be hard for the user to undo.

Since there's no strong reason to try to format an invalid or incomplete
file, we'll instead try parsing first and fail if parsing does not
complete successfully.

Since we talk directly to the HCL API here this is only a _syntax_ check,
and so it can be applied to files that are invalid in other ways as far
as Terraform is concerned, such as using unsupported top-level block types,
resource types that don't exist, etc.

There are no code changes in this update. This just catches up with the
latest master commit in the upstream repo, which has only seen
documentation updates since the prior commit.

This includes a missing feature for the splat syntax (null.* returns an
empty tuple) and also fixes a bug in the source code formatter where it
was inserting spaces between two consecutive interpolation sequences, like
"${foo}${bar}".

It's not normally necessary to make explicit type conversions in Terraform
because the language implicitly converts as necessary, but explicit
conversions are useful in a few specialized cases:

- When defining output values for a reusable module, it may be desirable
  to force a "cleaner" output type than would naturally arise from a
  computation, such as forcing a string containing digits into a number.
- Our 0.12upgrade mechanism will use some of these to replace use of the
  undocumented, hidden type conversion functions in HIL, and force
  particular type interpretations in some tricky cases.
- We've found that type conversion functions can be useful as _temporary_
  workarounds for bugs in Terraform and in providers where implicit type
  conversion isn't working correctly or a type constraint isn't specified
  precisely enough for the automatic conversion behavior.

These all follow the same convention of being named "to" followed by a
short type name. Since we've had a long-standing convention of running all
the words together in lowercase in function names, we stick to that here
even though some of these names are quite strange, because these should
be rarely-used functions anyway.

Bump provider installer protocol version to 5

Add labels to issue templates

states/statemgr: Avoid HTML escaping when printing LockInfo

The sethaselement, setintersection, and setunion functions are defined in
the cty stdlib. Making them available in Terraform will make it easier to
work with sets, and complement the currently-Terraform-specific setproduct
function.

In the long run setproduct should probably move into the cty stdlib too,
but since it was submitted as a Terraform function originally we'll leave
it here now for simplicity's sake and reorganize later.

In our new world it produces either a set of a tuple type or a list of a
tuple type, depending on the given argument types.

The resulting collection's element tuple type is decided by the element
types of the given collections, allowing type information to propagate
even if unknown values are present.

This hopefully makes it clearer that it works with sets rather than real
numbers, avoiding confusion with "sum", "min", "max", etc.

This creates the product interpolation function that returns the cartesian product of a list of lists.

Remove duplicate discovery logging

Fix shims with deeper nested containers

Require go1.11.4