Fix sys.path ordering: single-file module from earlier root beats package from later root

[kain88-de]

Summary

Python's sys.path semantics are first-match-wins regardless of whether the match is a .py file or a package directory. When widget.py exists in root0 and widget/init.py exists in root1, root0's module should win.

The bug was in FindResult::best_result, which unconditionally placed RegularPackage and LegacyNamespacePackage at the top priority tier. This meant a package from any fallback root would override a single-file module found earlier in the search path.

The fix adds an explicit match arm before the RegularPackage tier: when a is a SingleFilePyModule or CompiledModule and b is a package type, a wins. This preserves the existing behavior where .pyi stubs from later roots still beat .py implementations (handled by the subsequent SingleFilePyiModule arm).

Fixes #3407

Test Plan

Two new tests covering the bug directly:

• module::finder::tests::test_single_file_in_earlier_root_beats_package_in_later_root — unit
  test on find_module
• test::imports::test_search_path_module_beats_later_package — integration test verifying no
  type error when importing from the correctly resolved module

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