ROS plugins: source space not forced to be relative

[Kyle Fazzari]

The Catkin and Colcon plugins support remapping the source space from the default `src` to something else. However, while it's expected for it to be relative to the `source/source-subdir`, such an expectation is not enforced. If one were to use `source-space` as the absolute path to the source, it's accepted and builds a snap successfully, but certain features are broken (e.g. rebuilding). source-space should always be a relative path, and snapcraft should barf otherwise.