output: use path instead of relpath (#9673)

Relpath might not exist (e.g. on windows between different drives),
we should just use path as is and not get too fancy with the ui,
unless we want to constantly keep ValueError in mind when using
relpath for ui needs all over the place.

These operations should use index, where we always have current
directory and relative paths, so the ui will change accordingly
anyway.

.dvc/.gitignore

@@ -0,0 +1,3 @@
+/config.local
+/tmp
+/cache

dvc/output.py

@@ -20,6 +20,7 @@ from dvc.exceptions import (
     DvcException,
     MergeError,
 )
+from dvc.utils import format_link
 from dvc.utils.objects import cached_property
 from dvc_data.hashfile import check as ocheck
 from dvc_data.hashfile import load as oload
@@ -678,6 +679,14 @@ class Output:
     # pylint: enable=no-member
 
     def save(self) -> None:
+        if self.use_cache and not self.is_in_repo:
+            raise DvcException(
+                f"Saving cached external output {str(self)} is not supported "
+                "since DVC 3.0. See "
+                f"{format_link('https://dvc.org/doc/user-guide/upgrade')} "
+                "for more info."
+            )
+
         if not self.exists:
             raise self.DoesNotExistError(self)
 
@@ -1270,7 +1279,7 @@ class Output:
             raise FileNotFoundError(  # noqa: B904
                 errno.ENOENT,
                 os.strerror(errno.ENOENT),
-                self.fs.path.relpath(path),
+                path,
             )
 
         new = tree.from_trie(trie)
@@ -1393,8 +1402,7 @@ class Output:
         otransfer(staging, self.cache, {obj.hash_info}, hardlink=relink, shallow=False)
 
         if relink:
-            rel = self.fs.path.relpath(path)
-            with CheckoutCallback(desc=f"Checking out {rel}", unit="files") as callback:
+            with CheckoutCallback(desc="Checking out {path}", unit="files") as callback:
                 self._checkout(
                     path,
                     self.fs,