Split imports for directory strategy in buf generate

[oliversun9]

This is still WIP, but it seems to work with the base case.

Fixes #3056, the problem where only non-imports are split by directory in buf generate. The original issue has a great description and repro.

Note, this cannot be achieved by simply removing the import check here, because the split images will conflate IsImport with whether we want to generate a file if permitted by include-imports. The latter is introduced to a new type ImageFileForGeneration.

TODOs:

• add more detailed tests
• cleanup the code
• there are still places (*_test.go and .../alpha/protoc/plugin.go) that call ImageByDir, they should stop calling it.
• Now their are functions that call ImageByDir (the current implementation) and those that call ImageByDirSplitImports, perhaps we can completely get rid of ImageByDir.
• Perhaps move bufimagegenerate into bufgen

This also refactors some generation-specific functions/types into a new bufimagegnerate package, lots of moving around. I will add comments indicating which changes are new code and which ones are relocation.

[github-actions]

The latest Buf updates on your PR. Results from workflow Buf CI / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Aug 21, 2024, 5:54 PM

[bufdev]

@doriable didn't I have a partial PR for this? Is this using that?