Consider fetching manifests for direct dependencies only

[gavanderhoorn]

Suggested by @dirk-thomas: instead of having rosdistro_build_cache fetch the manifests of every package released (and listed in the distribution.yaml for a particular ROS release), calculate the transitive dependencies of a (set of) package(s) (using bits of code from probably rosinstall_generator) and download the manifests only for the packages in that set.

Depending on the size of the last set, this could significantly reduce the nr of manifests that need to be downloaded.

Calculating the complete set directly is probably impossible (as information from the manifests of layer n is needed to figure out which packages are part of layer n+1), so an iterative approach would be needed.