fix: support non-editable installs (site-packages import precedence)

[pythoniste]

When the package-under-test is installed in site-packages as a
non-editable wheel rather than via pip install -e ., mutmut's
trampoline-injected code in the mutants/ directory was never loaded.

Python imported the original (non-mutated) modules from site-packages
because they were already cached in sys.modules. The stats phase found
zero test coverage and all mutants were marked "not checked".

Add _patch_imported_packages() to setup_source_paths(): after adding
mutants/ to sys.path, patch path on every already-imported package
so submodule lookups prefer the mutants tree. Flush leaf modules whose
mutated .py exists on disk so they get re-imported with trampoline code.

Add _copy_parent_init_files() to copy_src_dir(): when paths_to_mutate
targets individual files, parent init.py files were missing from
the mutants tree, preventing Python from recognising it as a package.

Tested on a Pydantic-heavy project (8,996 mutants): stats phase now
finds 477 covered functions and achieves a 70% kill rate, where
previously it found zero coverage.

[pythoniste]

By the way, the branch name is misleading. At first, I wanted to fix Pydantic and then, I fixed something else.