Use `api.RepoName` and `reposource.PackageName` type aliases
Created by: olafurpg
Previously, we used strings for both types, which caused a regression that got fixed in https://github.com/sourcegraph/sourcegraph/pull/38722. This commit introduces type aliases to avoid regressions like that.
While implementing the refactoring I also found a bug in Python packages, that's fixed by this commit.
Test plan
See the CI go green.