Resolve generated stubs alongside editable packages

[ting-hong-shieh]

Summary

• preserve typed namespace roots when a later search root contains an editable regular package
• use those typed roots as overlays when resolving package submodules
• retain the regular package's __init__.py as the package module
• add resolver-level and end-to-end regressions for generated extension stubs

Root cause

Generated stubs installed next to an extension module form an implicit namespace package. When an editable source directory later supplied the package's __init__.py, normal regular-package precedence discarded the earlier namespace root, so submodules such as _core.pyi were no longer visible.

The overlay is limited to namespace roots containing py.typed, preserving existing behavior for ordinary implicit namespace packages.

User impact

Editable installs can expose their source package and generated extension stubs at the same time, so imported native APIs retain their declared types.

Testing

• cargo test module::finder::tests (71 tests)
• cargo test test_editable_install_generated_stub_overlay
• formatter and Clippy through test.py

Fixes #2859

[meta-codesync]

This pull request has been imported. If you are a Meta employee, you can view this in D115850824. (Because this pull request was imported automatically, there will not be any future comments.)

[ting-hong-shieh]

The Meta Linter check is currently reporting Action Required, but the internal details are not accessible to external contributors. Could someone with access share the failing diagnostic so I can address it? Thanks!