Make musetalk a proper Python package

Add missing __init__.py files so `musetalk.*` subpackages import
without relying on namespace-package behavior or sys.path hacks, and
narrow the root `/models/` gitignore so `musetalk/models/__init__.py`
is tracked.

Also add a minimal pyproject.toml so the source can be installed with
`pip install .` (or `pip install -e .`) without pulling the pinned
requirements.txt, which consumers typically need to override.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -5,7 +5,7 @@
 *.pyc
 .ipynb_checkpoints
 results/
-models/
+/models/
 **/__pycache__/
 *.py[cod]
 *$py.class