Move default sources into rosdep.

This patch moves the default sources into rosdep, and updates the search mechanism so that it searches for sources config files within the package, in /etc/ros/rosdep/sources.list.d, and in the user's home directory.

This patch also adds a post-install hook for debs that will remove the default sources from /etc/ros/rosdep/sources.list.d, if their md5sum matches any of the most recent 6 rosdistro revisions.

I've tested on my system and confirmed that the post-install hook works.

Unit tests are also updated appropriately.