Prevent BUCK files in dirsyncs (#2429)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2429

When I originally implemented this linter I wasn't sure about how to deal with dirsyncs, since then I've found the right solution (empty TARGETS files)

This updates the linter to:

1. Stop letting dirsynced BUCK files through
2. Enhance the warning for BUCK files in dirsyncs

Then I fixed the 2 cases that caused problems on my continuous testing runs.

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Reviewed By: zertosh, huydhn

Differential Revision: D54902249

fbshipit-source-id: 1ff18198e9ae3262b7e0730ccfc9b7623c52ff4a