Refactor default sources.

This moves the default sources into the package and updates the search
paths so that rosdep searches for sources in /etc/ros, $ROS_HOME, and
within the package itself.
Use isdir or isfile instead of exists.
Add migration hook to remove old sources.
Include sources in package.
clean up egg-info on 'make clean'
Oops; clean up debug code.
Remove groovy and hydro gbpdistro lines.
Implement searching for sources per discussion.
Note about gbpdistro on ROS distros after Fuerte.
Remove redundant if.
Installing and using default sources.
Cleanup.
Build default cache at build time.
Distributing cache with source.
Set cache to be world-readable.