Refactor ansible_galaxy.flat_rest_api.content into appropriate ansible_galaxy.*

[alikins]

ansible_galaxy.flat_rest_api.content.* is where most of the remnants of ansible-galaxy (upstream and the 'content' branch) remain.

That code should live elsewhere in the code base and ansible_galaxy.flat_rest_api.content removed, and eventually all of ansible_galaxy.flat_rest_api

The Content and InstalledContent classes need to be pared down to be attr based classes (see #126). Most of the rest of the code is implementation of installing roles content from archives and should be split up into smaller more focused pieces.

[alikins]

Fixed by #156