rf: use gitpathspec module from datasalad library

This module has been migrated from datalad-next to datasalad.

A deprecation warning it added to the module. All consuming code here
has been adjusted to the new imports.

datalad_next/gitpathspec/__init__.py

@@ -2,10 +2,20 @@
 
 This functionality can be used to add support for pathspecs to implementations
 that rely on Git commands that do not support submodule recursion directly.
-
 """
 
 __all__ = ['GitPathSpec', 'GitPathSpecs']
 
-from .pathspec import GitPathSpec
-from .pathspecs import GitPathSpecs
+import warnings
+
+from datasalad.gitpathspec import (
+    GitPathSpec,
+    GitPathSpecs,
+)
+
+warnings.warn(
+    '`datalad_next.gitpathspec` has been migrated to the datasalad library, '
+    'adjust imports to `datasalad.gitpathspec`',
+    DeprecationWarning,
+    stacklevel=1,
+)