Simplify package-template usage

This reduces the amount of changes necessary to customize the `package-template` for a new affiliated package.

It basically amounts to:

1. Updating some values in `setup.cfg`
2. Moving the main source directory

The important benefit of this is not that it takes less time to set up, but that this should make it easier to pull in changes to the `package-template` in the future.  (And in that regard, we probably want to start tagging package-template releases with each astropy release, but that's for another PR...)