Add openff-packmol-base output to existing openff-packmol feedstock

[mattwthompson]

I need to split this package into multiple outputs, using the fairly common foo-base approach. This is analogous to what I've done before in other packages, i.e. #1724. More context is at the link below, I believe everything is working except for the expected limitation, which brought me here

conda-forge/openff-packmol-feedstock#2

This is "my" feedstock, if desired I can solicit explicit approval from my other maintainer

Checklist:

• I want to add a package output to a feedstock:
  • Pinged the relevant feedstock team(s) (skipping - I am the team)
  • Added a small description of why the output is being added.