[FR]: fail the build when a write_source_files target is out-of-date

[alexeagle]

Currently write_source_files relies on a test target to inform developers that a file in bazel-out no longer matches the copy in the source tree, and prompting them with a bazel run command to perform the copy operation.

This isn't great because developers may be working with an outdated file in their editor, for example a newly added proto field isn't appearing for auto-completion. Only when they know to run tests (or when CI runs tests for them) do they learn of it.

However I believe that https://bazel.build/extending/rules#validation_actions allow us to catch this during bazel build instead. This makes it more likely developers find out - for example if they just bazel run //my:devserver they can get a build-time error that the schema is outdated.