validate_third_party_test: do not use main package

[nicdumz]

main makes it sound like this is a test for the main.go entrypoint and
this confuses Bazel automated BUILD file generation code.

Instead, create a test/ subpackage and run the test from there.

Notes:

• Code claimed to validate third_party/ templates but in practice was
  validating third_party/terraform only: clean up variables and comments
  to make this clearer.
• Because the test is moved one directory deeper, there is now a need
  for one more Dir() call to bubble one directory higher (relatively)
  to find ./third_party/ compared to the test file.

Tested: with local tests, and I also locally edited a template (into not a template) to verify test still fails as expected.

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[modular-magician]

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[melinath]

changing the package name is definitely fine, and the implementation changes are... probably fine? a couple nits/questions

[modular-magician]

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[melinath]

LGTM - I'm not sure we want this to live in a test directory long-term but I don't know what the best spot would be. Putting it next to main.go is definitely wrong, and third_party isn't a go package. So... this seems fine for now. We can always move it later if we want to.