Currently the Catkin plugin simply pulls down all dependencies of the workspace being built using rosdep. This doesn't work for the cases of trying to create a ROS system that utilizes content sharing, as it forces each snap to bundle their dependencies even when it's undesired. This PR resolves LP: [#1664776](https://bugs.launchpad.net/snapcraft/+bug/1664776) by adding support for an `underlay` property to the Catkin plugin, which accepts a build-time underlay path for when the consumer snap is being built (which is searched for dependencies before fetching them), as well as a run-time underlay path for when the consumer snap is being run (so the workspace can be properly overlaid). **Note:** This is the first of two PRs. The second adds a demo (and a snaps_test) that utilizes this. It might be helpful to see how this is used, though, so feel free to [check out the demo](https://github.com/kyrofa/snapcraft/tree/feature/1664776/catkin_underlay_snaps_tests/demos/shared-ros).