Prevent returning relative paths for a location

Our own override for the distutils implementation has a bug :)

src/pip/_internal/locations/__init__.py

@@ -111,15 +111,6 @@ def _fix_abiflags(parts: Tuple[str]) -> Iterator[str]:
         yield part
 
 
-def _default_base(*, user: bool) -> str:
-    if user:
-        base = sysconfig.get_config_var("userbase")
-    else:
-        base = sysconfig.get_config_var("base")
-    assert base is not None
-    return base
-
-
 @functools.lru_cache(maxsize=None)
 def _warn_mismatched(old: pathlib.Path, new: pathlib.Path, *, key: str) -> None:
     issue_url = "https://github.com/pypa/pip/issues/10151"
@@ -182,11 +173,9 @@ def get_scheme(
         prefix=prefix,
     )
 
-    base = prefix or home or _default_base(user=user)
     warning_contexts = []
     for k in SCHEME_KEYS:
-        # Extra join because distutils can return relative paths.
-        old_v = pathlib.Path(base, getattr(old, k))
+        old_v = pathlib.Path(getattr(old, k))
         new_v = pathlib.Path(getattr(new, k))
 
         if old_v == new_v:

src/pip/_internal/locations/_distutils.py

@@ -81,8 +81,14 @@ def distutils_scheme(
         scheme.update(dict(purelib=i.install_lib, platlib=i.install_lib))
 
     if running_under_virtualenv():
+        if home:
+            prefix = home
+        elif user:
+            prefix = i.install_userbase  # type: ignore
+        else:
+            prefix = i.prefix
         scheme["headers"] = os.path.join(
-            i.prefix,
+            prefix,
             "include",
             "site",
             f"python{get_major_minor_version()}",
@@ -91,10 +97,7 @@ def distutils_scheme(
 
         if root is not None:
             path_no_drive = os.path.splitdrive(os.path.abspath(scheme["headers"]))[1]
-            scheme["headers"] = os.path.join(
-                root,
-                path_no_drive[1:],
-            )
+            scheme["headers"] = os.path.join(root, path_no_drive[1:])
 
     return scheme