920322 - Use import_units() inside of _import_pkg_category_unit() to ensure that we handle package groups correctly.
Fix 920322 a different way, without recursion.
Add a unit test to assert that we copy and correctly alter
package_groups during import.
Update the docblock for _import_pkg_group_unit().
Merge pull request #151 from pulp/rbarlow-920322

Rbarlow 920322