Follow-up to #11057

As a follow-up to the temporary fix we made in #11057, we should do a proper fix by decoupling arg parsing from PR finder. @williammartin has already explained what needs to be done in the PR's description and the code.

Since this is a refactoring task, there should be no observable changes. Specifically, The affected/refactored pr subcommands (e.g. pr edit or pr view) must support PR URLs as argument.

Expected Outcomes

• Existing tests related to affected pr commands will still pass