This commit unifies the repo models from `package_repositories` and `projects`. Remarks: - The Apt, AptDeb, AptPPA and AptUCA classes are removed completely from the `projects` module. The verification that they performed is now done by their "duals" in `package_repositories`. - `projects.validate_repository()` is preserved as it's part of the "official" public API. Its implementation is changed to just call PackageRepository.unmarshal(), with (hopefully) no change in semantics. - The `PackageRepositoryApt.name` property is removed; as far as I can tell only test code used it and it's not documented (it looks like it's not meant to be used by client code), so might as well remove it while we're cleaning things up. - The majority of the model validation now generates Pydantic errors instead of the custom `errors.PackageRepositoryValidationError`. This is consistent with what the applications do but necessitates a small loss in "error clarity" because we remove the "resolution" and "details" fields. Hopefully the messages are still sufficiently clear. - There were many duplicated tests between `test_package_repository.py` and `test_projects.py`, because each set performed the same validation tests in the two model classes "types". Now the majority of the tests are in `test_package_repository.py`. - A minor point; the "priority" field is read as either an integer or one of the valid string values, but stored internally always as an integer (or None). - [ ] Have you followed the guidelines for contributing? - [ ] Have you signed the [CLA](http://www.ubuntu.com/legal/contributors/)? - [ ] Have you successfully run `tox`? ----- CRAFT-1665