Properly exclude files under "./vendor/" path, and find "files" only.
The original method "might" found "directories" with the same naming,
or, might exclude occasionally exclude the go files with "vendor" string
in any part of its path.

It seems that all of the tools we run here are now sufficiently
modules-aware to run without problems in modules mode, and indeed running
_not_ in modules mode was causing problems with locating packages in
mockgen.

The main significant change here is that the package name for the proto
definition is "tfplugin5", which is important because this name is part
of the wire protocol for references to types defined in our package.

Along with that, we also move the generated package into "internal" to
make it explicit that importing the generated Go package from elsewhere is
not the right approach for externally-implemented SDKs, which should
instead vendor the proto definition they are using and generate their
own stubs to ensure that the wire protocol is the only hard dependency
between Terraform Core and plugins.

After this is merged, any provider binaries built against our
helper/schema package will need to be rebuilt so that they use the new
"tfplugin5" package name instead of "proto".

In a future commit we will include more elaborate and organized
documentation on how an external codebase might make use of our RPC
interface definition to implement an SDK, b...

We're still using vendoring for now until we get _all_ of our tooling
updated, so the main idea here is to force use of the vendor directory
when running tests and building for development so we can quickly find
situations where we forget to run "go mod vendor".

We also setting GO111MODULE=off for installation of tools. Right now this
is the best way to install a tool in GOBIN without also interfering with
go.mod and go.sum, until a better pattern for managing tool dependencies
is devised by the Go team.

Finally, we run "go mod download" before launching "gox" in the main
build process, to prime the local module cache once so that the concurrent
"go build" processes won't race to populate it redundantly. This means
that we'll be producing final builds from the module cache rather than
from vendor as with everything else -- there's currently no way to tell
gox to use -mod=vendor -- but that should be fine in practice since
our go.sum file will ensure that we ...

Since protoc is not go-gettable, and most development tasks in Terraform
won't involve recompiling protoc files anyway, we'll use a separate
mechanism for these.

This way "go generate" only depends on things we can "go get" in the
"make tools" target.

In a later commit we should also in some way specify a particular version
of protoc to use so that we don't get "flapping" regenerations as
developers work with different versions, but the priority here is just to
make "make generate" minimally usable again to restore the dev workflow
documented in the README.

This also includes some updates that resulted from running "make generate"
and "make protobuf" after those Makefile changes were in place.

The test target will already cover vet, since it's run as part of the
test command now.

Also remove the `go test -i` since it's no longer needed with the new
build cache.

Remove the vet target too, since vet is now always run with every test.

We don't usually run "acceptance tests" during a Travis run, but this
particular suite doesn't require any special credentials since it just
accesses releases.hashicorp.com to download plugins, so therefore it's
safe to run in Travis at the expense of adding a few more seconds to
the runtime.

Running it in Travis can therefore give us some extra confidence for
pull requests that may inadvertently break certain details of the
workflow, as well as ensuring that these tests are kept up-to-date as
the system changes.

As of Go 1.9, ./... excludes the vendor directory by default and so we
no longer need to jump through hoops to exclude vendored packages from
testing, vetting, etc.

As a simplification this also re-introduces builtin/bins to the set of
packages we run tests on. With the providers now split into their own
repositories there's far fewer of these and so including them doesn't
really hurt anything, and makes our invocations here simpler.

Since there is little left that isn't core, remove the distinction for
now to reduce confusion, since a "core" binary will mostly work except
for provisioners.

The existing harness was only for aws, which fails now that aws is gone.

* Use latest in go 1.8 branch for travis

* Fix ongoing vet issues
Move vet step after the testing step in travis
Correct Makefile vet step to check correct files

updating types + tests

Increase timeout to 60s. The consul backend tests come too close to that
and timout every time they stall a little.

Running `go test -i` installs the requirements for a package's tests.
This way when running the tests in batches of 4, we can cut the runtime
in half be only compiling the dependencies once.

We were running out of RAM on Travis

Have vendor-status run govendor-status
Add all the missing .PHONY targets

The vet target was only vetting the topmmost packages.

* provider/aws: Add errcheck to Makefile, error on unchecked errors

* more exceptions

* updates for errcheck to pass

* reformat and spilt out the ignore statements

* narrow down ignores

* fix typo, only ignore Close and Write, instead of close or write

Allows specifying `LD_FLAGS` during development builds.

```
$ LD_FLAGS="-linkmode=external" make dev
```

Since the version of DTrace on macOS Sierra (On the public beta's at least) has been updated, this allows DTrace to run on Terraform during development/debugging.

```
$ sudo dtrace -n 'pid$target::main.main:entry' -c "terraform apply"
dtrace: description 'pid$target::main.main:entry' matched 1 probe

Apply complete! Resources: 0 added, 0 changed, 0 destroyed.

Outputs:

foo = bar
dtrace: pid 23096 has exited
CPU     ID                    FUNCTION:NAME
  2 265673                  main.main:entry
```

Also redirects the `@which stringer` command inside the `generate` Makefile target to `/dev/null` so we stop echoing the path to the `stringer` binary on every call to `generate`.

This is intended to reduce cycle time during provider development.

This is time consuming and spams the output of Travis - to run no tests.

Several of our vendered dependencies are not gofmt-compliant, but we don't
want to fix that since the vendored code is supposed to exactly match
upstream.