Improve 'alias_of' handling in wp_insert_term() and wp_update_term().

Using get_term_by() rather than direct SQL queries to fetch the alias term
fixes a number of issues:

* Object cache for aliased term is properly cleared after update.
* If the aliased term is in the object cache, it's served from there, saving a database query.
* Duplicate 'edit_terms' and 'edited_terms' hooks can be removed.
* Fix a PHP notice when the 'alias_of' term is not found.
* Prevent the incorrect creation of a new term group for the primary term when the 'alias_of' term is not found.

Adds unit tests for 'alias_of' functionality in both functions.

Fixes #29848.

git-svn-id: https://develop.svn.wordpress.org/trunk@29862 602fd350-edb4-49c9-b593-d223f7449a82