Fix dataset query missing implied dimensions

Revert serialization changes in SerializedDatasetType and SerializedDataCoordinate, which caused them to drop implied dimensions.

This leads to an incorrect definition of the dataset type after deserialization, since it is missing some dimensions.  It also breaks the transfer of DatasetRefs from server to client, since it drops the values for implied dimensions in the data ID.  Per the documented behavior of dataset queries, the returned refs will have `hasFull() = True` and this behavior is relied on in tutorial notebooks.

python/lsst/daf/butler/_dataset_type.py

@@ -676,7 +676,7 @@ def to_simple(self, minimal: bool = False) -> SerializedDatasetType:
                 "name": self.name,
                 "storageClass": self._storageClassName,
                 "isCalibration": self._isCalibration,
-                "dimensions": list(self._dimensions.required),
+                "dimensions": list(self._dimensions.names),
             }
 
             if self._parentStorageClassName is not None:

python/lsst/daf/butler/dimensions/_coordinate.py

@@ -709,7 +709,7 @@ def to_simple(self, minimal: bool = False) -> SerializedDataCoordinate:
         else:
             records = None
 
-        return SerializedDataCoordinate(dataId=dict(self.required), records=records)
+        return SerializedDataCoordinate(dataId=dict(self.mapping), records=records)
 
     @classmethod
     def from_simple(

python/lsst/daf/butler/tests/butler_queries.py

@@ -303,6 +303,18 @@ def test_simple_dataset_query(self) -> None:
             butler.query_datasets("bias", "*", detector=100, instrument="Unknown", find_first=False)
         self.assertIn("doomed", str(cm2.exception))
 
+        # Test for a regression of an issue where "band" was not being included
+        # in the data ID, despite being one of the dimensions in the "flat"
+        # dataset type.
+        refs = butler.query_datasets("flat", "imported_r", where="detector = 2", instrument="Cam1")
+        self.assertEqual(len(refs), 1)
+        flat = refs[0]
+        self.assertEqual(flat.datasetType.name, "flat")
+        self.assertEqual(flat.dataId["instrument"], "Cam1")
+        self.assertEqual(flat.dataId["detector"], 2)
+        self.assertEqual(flat.dataId["physical_filter"], "Cam1-R1")
+        self.assertEqual(flat.dataId["band"], "r")
+
     def test_general_query(self) -> None:
         """Test Query.general and its result."""
         butler = self.make_butler("base.yaml", "datasets.yaml")