[infra] Add some logic for checking excluded g3 things

[dcharkes]

We don't roll everything into g3, because not everything functions in g3:

• findPackageRoot doesn't work in the g3 test runner
• package:json_syntax_generator and package:json_schema are not rolled in.

In order for tests to not fail in the g3 roll, the following directories are either excluded from the copy, or excluded from the test targets per package:

• example/
• test/json_schema/
• test/example/
• test/locking/locking_test.dart
• test/test_test.dart (hooks)

For reference, either in unbundle_config.bzl (for copying) or the BUILD file of the package.

It would be nice if we would write a custom analyzer plugin or some script that can check this before the google3 roll fails. On idea: It would be nice if we could annotate things with @noG3, auto exclude those from the roll and targets, and lint/warn on referring to definitions annotated with @noG3 referenced in files that are not excluded.

FYI @liamappelbe Thanks for skipping the g3 test for now. Here's a breakdown of how other tests are excluded.

[dcharkes]

@iinozemtsev's idea: use a .g3ignore. Using annotations would be complicated because it would require analyzing the code (or at least regex-ing with potential false positives) as a complicated step in g3.

Or we do something like ignoring test/no_g3/, but that forces the test organization to be wonky.