List names, demucs cleanup

[zdevito]

Stack from ghstack:

• #161 [wip] some clean and code to package models

• #158 List names, demucs cleanup

• Separate listing the names of modules from actually loading one.

• Module import for a model sometimes takes time, so it is helpful to be able to filter by module name before importing the module souce.

• Also allows manually looking up module load_model('demucs')

• Fixes an issue where demucs get_module was not returning a module.