Discover and prefer the parent interpreter when invoked with python -m uv (#3736)

Closes #2222
Closes https://github.com/astral-sh/uv/issues/2058
Replaces https://github.com/astral-sh/uv/pull/2338
See also https://github.com/astral-sh/uv/issues/2649

We use an environment variable (`UV_INTERNAL__PARENT_INTERPRETER`) to
track the invoking interpreter when `python -m uv` is used. The parent
interpreter is preferred over all other sources (though it will be
skipped if it does not meet a `--python` request or if `--system` is
used and it belongs to a virtual environment). We warn if `--system` is
not provided and this interpreter would mutate system packages, but
allow it.

python/uv/__main__.py

@@ -31,6 +31,9 @@ def _run() -> None:
     if venv:
         env.setdefault("VIRTUAL_ENV", venv)
 
+    # Let `uv` know that it was spawned by this Python interpreter
+    env["UV_INTERNAL__PARENT_INTERPRETER"] = sys.executable
+
     if sys.platform == "win32":
         import subprocess