Debt: unify repository/branch/pr open workflows into common place

[axosoft-ramint]

Flows from launchpad, launchpad view, PR view, etc. all go through different flows for getting to targets. Deep link service, pullRequest.ts, openOrCreateWorktree.ts for the PR extension, all have different ways of getting to a target repository, locating if needed, adding remote, fetching, etc. and ultimately getting to the target (PR branch, branch, commit, etc.).

All of this very-similar logic should be unified in one place, and everything should go through the common logic.

pullRequest.ts is a good model since it leverages virtual repo opening and other tricks to locate the repo.