Support git repo detection for libraries in make-generate

Problem/Motivation

As more and more libraries are available as git repo it would be great to have the same git detection for them as for modules and themes in drush make-generate.

Proposed resolution

Encapsulate and reuse the code with the git repo detection.
I moved parts of the code from _drush_generate_custom_project() into a dedicated function called _drush_generate_get_download_settings().
I'm not to happy with the mix of referenced parameters and a return value - ideas welcome.
The new function is used in _drush_generate_custom_project() for projects and in _drush_make_generate_projects() for libraries.

Remaining tasks

Reviews needed

User interface changes

None

API changes

None