Migrates all legacy code for the `upload-metadata` command. No behavioral changes. This is as pure of a refactor as I could manage. I made minor changes like updating from `logger` to `craft_cli`, adding type hints, and clarifying docstrings. That means there are plenty of inefficiencies and bugs. For example, you'll see places where we access dict values without guarding against KeyErrors or error messages that begin with "Sorry". I left all this code as-is to limit what reviewers need to focus on (the migration, not the implementation). Otherwise, this review would take forever. The unit tests had to be completely rewritten. They were using `unittest` and a bit of a mess due to their age and neglect. Unfortunately, there was very little code that I could remove from `snapcraft_legacy`, as the code is still used for other store commands still in snapcraft_legacy. I recommend reviewing this by commit. Partially completes #5338 (SNAPCRAFT-764) --- - [x] I've followed the [contribution guidelines](https://github.com/canonical/snapcraft/blob/main/CONTRIBUTING.md). - [x] I've signed the [CLA](http://www.ubuntu.com/legal/contributors/). - [x] I've successfully run `make lint && make test`. - [ ] I've added or updated any relevant documentation. - [ ] I've updated the relevant release notes.