Normsep Paths in taskgraph.util.hash

On windows str(Purepath()) is used for matching the pattern
which will match nothing, given `\\` is in the string.

src/taskgraph/util/hash.py

@@ -55,4 +55,4 @@ def _find_matching_files(base_path, pattern):
 
 @memoize
 def _get_all_files(base_path):
-    return [str(path) for path in Path(base_path).rglob("*") if path.is_file()]
+    return [mozpath.normsep(str(path)) for path in Path(base_path).rglob("*") if path.is_file()]