Fix isolated build failures and missing rospack dep

This PR hopes to solve two issues:

The first is that catkin must be `find_package`'d when using an isolated workspace. To reproduce this bug, simply compile `cob_extern`'s packages using catkin_make_isolated.

The second is that `rospack` is not necessarily present on every system, and since it is used to find `mk`, it needs to be in the `build_depend` list. To reproduce this, simply uninstall `rospack`, use `rosdep` to verify that the build_depends are met, and try to compile `cob_extern`'s packages.

I found another issue that I should mention here, but it is a problem in `rospack`, not here. See https://github.com/ros/rospack/pull/36.

Since this is a complete blocker for building on Fedora, I would appreciate a backport of this to groovy.

Thanks!

--scott