Remove the need for the `repos.json` input file

[lzkelley]

Perhaps an individual catalog doesn't need to use the output-repository structure that supernovae does. For example, the catalog may not version-track the output files at all, they may all be contained in a single repository, or perhaps they want to keep the output in the same repository as the code itself.

Instead of requiring the repos.json file and associated methods, we can just require the necessary API methods. i.e. instead of get_repo_output_file_list() (which is used by the delete_old_entry_files() method --- and associated task), require a more general get_output_file_list() which can store/construct those files in any (reasonable) way they wish.