fix: support src-layout Python projects in test_coverage detector

[AreboursTLS]

Problem

In Python projects using src/ layout (where production code lives under src/package_name/), the test_coverage detector fails to link test files to source files.

resolve_import_spec converts import specs like mypackage.foo to mypackage/foo.py, but production files are stored as src/mypackage/foo.py. The candidates never match.

_build_prod_by_module creates module aliases with a src. prefix (e.g. src.mypackage.foo) that import specs from tests never include.

This causes all modules in src-layout projects to be flagged as untested_critical even when comprehensive test suites exist that import them.

Fix

1. resolve_import_spec (languages/python/test_coverage.py): After checking direct candidates against production_files, also try src/-prefixed candidates.
2. _build_prod_by_module (engine/detectors/coverage/mapping_analysis.py): Strip src/ prefix from relative paths before computing module names, so the index maps argos_toolkit.foo instead of src.argos_toolkit.foo.

Testing

• All 5495 existing tests pass.
• Verified against a real src-layout project (argos-toolkit): test health detection went from broken (0%) to correctly detecting 22.0% coverage.

[peteromallet]

Thanks for the fix, @AreboursTLS! The src-layout gap is real — PEP 621 projects with src/package_name/ structure were getting missed by the coverage mapping.

Cherry-picked both changes into 0.9.12 as commit 36805343:

• resolve_import_spec now tries src/-prefixed candidates when direct match fails
• _build_prod_by_module strips the src/ prefix before computing module names

One minor adjustment: moved _SRC_PREFIXES to module level as a constant (was function-local in the PR).

All tests pass. Thanks for the clean, targeted fix!