build(gix): preserve the extra in local source distributions

Include `gix-requirements.txt` explicitly alongside the other requirement
files so source distributions can resolve the optional backend with every
supported setuptools version. Keep tox's wheel builder in `.pkg-gix`, whose
package installation is restricted to the local wheel directory.

Built a source archive locally, verified that it contains the new requirements
file, and rebuilt its wheel without accessing a package index. The rebuilt
wheel declares `Provides-Extra: gix` and the conditional `GixPython`
dependency. Tox itself was not available locally; the interpreter-based test
runner and its offline installation test were exercised instead.

Author: codex

MANIFEST.in

@@ -5,6 +5,7 @@ include LICENSE
 include README.md
 include VERSION
 include requirements.txt
+include gix-requirements.txt
 include test-requirements.txt
 include git/py.typed
 

tox.ini

@@ -28,6 +28,7 @@ set_env = GIT_PYTHON_BACKEND=gix
 description = Run the suite with released GixPython and CLI fallback
 base_python = python3
 package_env = .pkg-gix
+wheel_build_env = .pkg-gix
 extras = test, gix
 commands = python test/run-local.py {posargs:--backend-report=.cache/gix-coverage.json}