Consider repos which already have build recipes

Some repos ship with a conda recipe as part of the source - it would be nice to avoid duplicating the recipe, and just use that directly - perhaps using git submodules (which are horrible, but would work for this process). Updating a recipe would then be a matter of updating the submodule location.

I did something similar in https://github.com/pelson/package_with_continuous_delivery/tree/_build.

This might also give us a way to package packages for which no recipe is needed (because the `conda skeleton` approach works).