Do not remove generated files from existing rules

[gg-sr]

Before, generated sources were removed from srcs and hdrs; rules with only generated sources would be removed as well.

With this PR, srcs and hdrs keep their generated sources, and rules with only generated sources are kept.

This also seems enough for Gazelle to add dependencies to targets when generated headers are included (e.g. in the example added in this PR, a dependency from test to alone is correctly added based on #include "generated/alone.h").

I decided to only preserve existing srcs and hdrs, rather than adding new ones (e.g. by adding generated sources to fileInfos). I figured treating generated files as sources files would be nice, but would likely have more edge cases.

I cooked this up pretty quickly. I guess there are better ways to add tests than modifying example/bzlmod, but this was the fastest way to test things (and worked with the debugger).

[jayconrod]

Thanks for sending this fix.

A few comments. Please also add Signed-off-by: Your Name <your@email.com> to all commit messages (use git commit -s).

[gg-sr]

Thank you for the review!

I had to force push to change the commit message with --signoff. I made commits for subsequent changes with the expectation that they'd all be squashed before merging into main.

Notably I added helpers to debug tests, similar to how we can debug examples. I can revert them, but I figure they could be useful to others.

Btw, I tried to add support for :generated_file.h syntax as well, but it seems non-trivial (removing the : prefix is part of the solution, but not enough) so I just dropped it.

[jayconrod]

As mentioned in the other PR, CI is failing because of a misconfiguration on my side: we enabled remote caching, but the secret needed to make that work isn't available on PRs from external contributors.

I've verified tests pass locally, so that's good enough for now.