Use the "venv" scheme if available to obtain prefixed lib paths

get_prefixed_libs() computes the Python path for libraries in a pip
isolation environment. Python 3.11 introduced the "venv" path scheme
to be used in these cases. Use it if available.

This solves a bug on Homebrew's Python 3.10 and later where the
default paths scheme when Python is invoked outside a virtual
environment is "osx_framework_library" and does not relative to the
"{base}" or "{platbase}" variables.

Fixes pypa#11539.

news/11598.bugfix.rst

@@ -0,0 +1 @@
+Use the "venv" scheme if available to obtain prefixed lib paths.

src/pip/_internal/locations/_sysconfig.py

@@ -214,5 +214,9 @@ def get_platlib() -> str:
 
 
 def get_prefixed_libs(prefix: str) -> typing.Tuple[str, str]:
-    paths = sysconfig.get_paths(vars={"base": prefix, "platbase": prefix})
+    vars = {"base": prefix, "platbase": prefix}
+    if "venv" in sysconfig.get_scheme_names():
+        paths = sysconfig.get_paths(vars=vars, scheme="venv")
+    else:
+        paths = sysconfig.get_paths(vars=vars)
     return (paths["purelib"], paths["platlib"])