Search system paths for native binaries

Previously, each <package-implementation> had to give the absolute path of
the main binary. This causes unnecessary duplication e.g.

  <package-implementation package="graphviz" distributions="RPM Debian Arch" main="/usr/bin/dot"/>
  <package-implementation package="graphviz" distributions="MacPorts" main="/opt/local/bin/dot"/>
  <package-implementation package="graphviz" distributions="Ports" main="/usr/local/bin/dot"/>

Now, if the main value isn't found, we search the other system paths for the
binary. We also now allow non-absolute values (e.g. main='dot'). However, that
will break older versions of 0install and so should be avoided for now.