hydra: Fix sweeps on Defaults List.

Defaults List overrides (`defaults/foo=1,2`) were not working because of the inclusion of the `/` character.
Fixed it by updating how we construct the `merged_overrides` and added a regression test.

tests/func/utils/test_hydra.py

@@ -183,6 +183,13 @@ def test_compose_and_dump(tmp_dir, suffix, overrides, expected):
 @pytest.mark.parametrize(
     "overrides, expected",
     [
+        (
+            {"params.yaml": ["defaults/foo=1,2"]},
+            [
+                {"params.yaml": ["defaults/foo=1"]},
+                {"params.yaml": ["defaults/foo=2"]},
+            ],
+        ),
         (
             {"params.yaml": ["foo=1,2", "bar=3,4"]},
             [