Standardize on install_dir

Sorry for the confusion folks, but it seems like the move to install_path was
an unwise one. It is impossible to retain backwards compatability using
install_path, and all the other methods in omnibus use the "_dir" suffix,
especially those in the Config object.

This commit is basically a revert of c4c604b, but has some additional fixes
thus making it not a real "revert".
Expose Software#dependencies as part of the public DSL API
Expose Project#dependencies as part of the public DSL API
Rename install_path_cache_dir to git_cache_dir

The naming of this method is super confusing to new users. We should call it
what it is - the git_caching dir.
Remove Mixlib::Config. Evaluate Config in a cleanroom.