Make _gen_new_index(repodata, subdir) more modular

[msarahan]

• I read the conda-forge documentation and could not find the solution for my problem there.

Issue:

The main patching function has become pretty large. It might be nice to break it up into one file per patch, like a plugin architecture. It would really help minimize diffs on things like #215

Environment (conda list):

$ conda list

Details about conda and system ( conda info ):

$ conda info