Resolve absolute imports under the src/ layout (and recognize Sphinx conf.py)

[willfrey]

Problem

Two false positives in orphaned-file detection (and the import graph that feeds coupling / single-use / facade detection) for standard Python project structures.

1. Absolute imports don't resolve under the src/ layout

resolve_absolute_import only tries <scan_root>/<pkg> and <project_root>/<pkg>. In the PyPA-recommended src/ layout the importable package lives at <root>/src/<pkg>, which is never tried. So an absolute import like from pkg.sub import x resolves to nothing, the imported module is recorded with zero importers, and:

• it is misreported as an orphaned file, and
• every other graph-based detector (coupling, single-use, facade) silently loses those edges.

Relative imports (from .sub import x) were unaffected, which is why this only bites packages imported via their absolute path. On one real-world src/-layout project, scanning produced ~70 false findings that disappeared once edges resolved (6 bogus "orphaned file" reports plus ~64 downstream across the other detectors).

2. Sphinx conf.py is flagged as orphaned

docs/conf.py is loaded by sphinx-build and has zero importers by design, but it was not in PY_ENTRY_PATTERNS.

Fix

• Factor the candidate roots into candidate_source_roots() and add the src/ variants, tried after the existing flat roots. The change is strictly additive: any import that resolved before resolves to the same file; only previously-unresolved src/-layout imports gain an edge. Duplicate roots (common when scan root == project root) are collapsed.
• Add conf.py to PY_ENTRY_PATTERNS, alongside the existing manage.py / wsgi.py / settings.py framework entry points.

Tests

• test_absolute_import_resolves_under_src — constructs a src/-layout package and asserts the imported module's importer_count >= 1. Red before the fix (importer_count == 0), green after.
• test_sphinx_conf_py_excluded_by_python_entry_patterns — feeds the real PY_ENTRY_PATTERNS through detect_orphaned_files and asserts a docs/conf.py is not flagged while a genuine dead file still is.
• Full Python-language suite (268 passed) and the orphaned-detector suite pass.