This PR have a lot of copy/pasted and automated fixes, so I strongly suggest looking at the individual commits: 1) The first commit is a literal copy-and-paste for the `snapcraft.repo` package and the tests in `tests/unit/repo`; 2) The second commit is a find-and-replace of "snapcraft" to "craft_archives". Note that I kept used filenames like `snapcraft.gpg` unchanged because this part of the code will be changed in the near future, and changing it now would break snapcraft; 3) The third commit has actual "manual" changes. These include using loggers instead of craft_cli's `emit()`, adding a base error class and copying the `os_release` and `util` utility packages from snapcraft. 4) The final commit has changes to address linting errors, because craft-archives uses `ruff` and `pyright` in stricter settings. CRAFT-1549