Fix relative import resolution in namespace packages

[TheGreatApollyon]

Description

When a module is part of a namespace package (a directory without an init.py), Module.relative_to_absolute_name used to drop the namespace package prefix.

For example, with this layout:

pkg/
├── main.py          # from .subpkg import mod
└── subpkg/
    └── mod.py

where pkg is a namespace package on sys.path, from .subpkg import mod in pkg.main was resolved as subpkg instead of pkg.subpkg, raising AstroidImportError: Failed to import module subpkg.

Fix

Keep the namespace package prefix (derived from the module name) when the relative import target lives in the same directory as the module. Modules whose name carries no package prefix (e.g. built from a file not on sys.path) keep the previous behavior.

Closes #2766

[codspeed-hq]

Merging this PR will not alter performance

✅ 3 untouched benchmarks
⏩ 1 skipped benchmark¹

---

_{Comparing TheGreatApollyon:fix/issue-2766-namespace-relative-import (b506576) with main (2beb35b)}

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩

[codecov]

Codecov Report

❌ Patch coverage is 88.46154% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.51%. Comparing base (3aacc67) to head (b506576).
⚠️ Report is 38 commits behind head on main.

Files with missing lines	Patch %	Lines
astroid/brain/brain_re.py	88.00%	3 Missing ⚠️

❌ Your patch check has failed because the patch coverage (88.46%) is below the target coverage (100.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3178      +/-   ##
==========================================
- Coverage   93.53%   93.51%   -0.03%     
==========================================
  Files          93       93              
  Lines       11480    11502      +22     
==========================================
+ Hits        10738    10756      +18     
- Misses        742      746       +4     

Flag	Coverage Δ
linux	93.38% <88.46%> (-0.03%)	⬇️
pypy	93.51% <88.46%> (-0.03%)	⬇️
windows	93.48% <88.46%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
astroid/nodes/scoped_nodes/scoped_nodes.py	93.20% <100.00%> (-0.08%)	⬇️
astroid/brain/brain_re.py	93.33% <88.00%> (-6.67%)	⬇️

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[DanielNoord]

CI still fails.