push-dests: move the code around missing default dest inside `get_push_paths` This function has a clear semantic and moving the code dealing with this inside it will help reduce duplication. Differential Revision: https://phab.mercurial-scm.org/D10385

push-dests: rework the handling of default value This new core is more straightforward and doing this early will make the next changeset simpler. Differential Revision: https://phab.mercurial-scm.org/D10384