fix: getAspectClassNames to handle union alias

[ybz1013]

Summary

Small fix of getAspectClassNames to handle union alias, to unblock EbeanLocalRelationshipQueryDAO.findEntities. (related to recent change in #436

The buggy behavior was that when aliases exists in the union type, this function will return the alias instead of the canonical name.

For example:

typeref EntityUnionAlias = union[
  foo: EntityFoo,
  bar: EntityBar
]

old code will return foo when we actually expects com.linkedin.testing.EntityFoo

Testing Done

added unit test
./graldew build

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Docs related to the changes have been added/updated (if applicable)

[jsdonn]

Thanks for proactively fixing the bug