CDK-633: Add simpler Hive URI patterns.

This updates Hive's URIs so that the following are valid:
* dataset:hive:name (default namespace)
* dataset:hive:namespace/name
* dataset:hive://ms-host/name (default namespace)
* dataset:hive://ms-host/namespace/name

These URIs were previously valid external URIs, but have been
repurposed. This is safe because the new patterns only match one or two
path components. Existing absolute path URIs are still valid and behave
the same way (the tests have not changed). The relative URI patterns
still load the correct dataset because existing datasets must be either
default.name or namespace.name in the Hive MetaStore. The URIs that have
changed are tested in TestHiveDatasetURIsCompatibility. The URI patterns
are tested, but the host to metastore URI is not because that is covered
by existing tests.

This required adding namespace backward compatibility (checking the
defualt namespace when no table is found) to the managed provider. The
implementation was to merge the working methods from the external
provider into the abstract provider where they are used for both managed
and external.