tag: enforce unique tag inheritance names and priorities client-side

[ktdreyer]

The koji_tag module calls a validate_repos() method to ensure that the playbook author has specified valid inputs for external_repos. The playbook author must specify a unique name string and priority integer for each external repository. This method will raise DuplicateNameError or DuplicatePriorityError if the user specifies invalid inputs.

We should do the same validation check for the tag inheritance values.

This will help us catch errors earlier when we apply tag inheritance in check mode. This is important when a playbook might assemble the final tag inheritance value from a couple different variables, and the final impact is not immediately clear when a user changes one variable without looking at all the others.

Note: thankfully the Koji Hub will raise GenericError: Inheritance priorities must be unique or GenericError: Changes should not contain duplicated parent_id if the user specifies invalid inputs live, so this is already solved for "live" mode on the server-side. We just need to implement the validation client-side to improve the check mode UX.

Second note: I've updated the server-side message text to be clearer, at https://pagure.io/koji/pull-request/3208, and discussed adding DuplicatePriorityError in https://pagure.io/koji/pull-request/3152