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.
clean up egg-info on 'make clean'
Implement searching for sources per discussion.
Installing and using default sources.
Set cache to be world-readable.