Default src.root to ['.', '<project_name>'] if the directory exists

[MichaReiser]

Summary

I noticed in #18137 that we fail to resolve all imports for some ecosystem repositories because they don't use the "traditional" src or flat layouts. Instead, they use a src layout but they use the project name instead of src for the "src folder name".

An example is psycopg where all source files are in psycopg/psycopg/_adapters_map.py:279:14

This PR extends our heuristics for src.root to recognize if a project has a <project>/<project> folder (and no src folder) and, if so, includes it in the src.root.

The motivation for this change is to support more projects with zero configuration.

I don't have a good sense of how common this project layout is, but it seems worth supporting, given that multiple ecosystem projects use it. This also makes our mypy primer results significantely more useful :)

Test plan

See mypy primer results, added unit tests.

[MichaReiser]

and mypy primer hangs here too :( I wonder if it is just because of the too large diff. Any suggestions on how I could try this out?

[github-actions]

mypy_primer results

No ecosystem changes detected ✅

[MichaReiser]

• Found 797 diagnostics

• Found 646 diagnostics

There are no other changes to mypy primer, so maybe it's not worth changing the default just for one project. I'd be interested to hear from people with more knowledge of the Python ecosystem to hear how common this project structure is.

[MichaReiser]

I think we should fix this for this specific mypy primer project for now. We can consider changing our heuristics if this comes up more frequently.

[AlexWaygood]

This proposed change makes a lot of sense to me. The src/ convention is encouraged by most common package managers, but it's a fairly recent convention. Until relatively recently, it was very common to have the directory structure that this PR adds support for, and it's still not uncommon for older projects.

I think we should also add tests/ to the src.root default, as discussed in astral-sh/ty#414 (comment)

[carljm]

I also wouldn't have any objection to resurrecting and landing this PR; it doesn't seem to have any downside.

I'm not sure it's necessary that we check that <project-name>/<project-name> exists, as this PR currently does? That is, it would be fine to do this simply if <project-name>/ exists.

[carljm]

I'm not sure it's necessary that we check that <project-name>/<project-name> exists, as this PR currently does? That is, it would be fine to do this simply if <project-name>/ exists.

Never mind this, Alex pointed out that if just <project-name>/ exists, it might be a flat layout, and we probably shouldn't add the extra root in that case.

[MichaReiser]

I think we should also add tests/ to the src.root default, as discussed in astral-sh/ty#414 (comment)

I'm up for this but I suggest doing this in a separate PR. While related, it needs to apply to all layouts.

[AlexWaygood]

sounds good