Detect packer/yaourt in get_install_command(), not at init.
Added support for detecting packages that "provide" other names.

Now rosdep will accept, e.g., python2-rospkg-git for python2-rospkg. I couldn't
find any way to get pacman to search for packages that match a "provides"
query, and I didn't want to depend on another library, so I added a function
that parses the /var/lib/pacman/local/$package/desc files.