make PFS pipeline installable with eups distrib install

As per discussion with Robert and John it is fine for now to install drp_stella, drp_stella_data, and obs_pfs from github, but in the near future we would want the installation be possible via eups distrib install.

Note that this doesn't preclude pulling some packages from git, the two systems work well together. We should expect to have a complete copy of the PFS code installed at all major sites and usable by people without having to do an install; eups distrib install provides a way to install updated components into this central copy. Because the PFS dependencies are simple, this is really pretty easy to do (the base LSST stack can be transparently installed as part of this operation).