[RFC] Decouple mappings from main pipreqs release cycle?

Many of the changes in `pipreqs` end up being additions or alterations to the mapping file. In the interest of keeping mappings up to date and releasing changes to them more easily, it may make sense to provide mappings as a separate set of information via another library (with more regular and frequent releases) or even a file hosted somewhere (this could be the current location in the git repository) coupled with an operation like `pipreqs update-mappings`.

Start by reviewing the current mapping file and how pipreqs packages and releases it. Compare the proposed separate library, hosted file, and `pipreqs update-mappings` approaches; done means selecting and implementing a clear decoupling design with a workable mapping update path.

Written by the indexing model from the issue text.