[Request] Create `ohmg.core.importers` module to support broader map ingestion capabilities

[mradamcox]

Describe the feature or enhancement you're looking for

To support our collaboration with the Black Butterfly project, we need to ingest maps for georeferencing that are not in the LOC collection, nor served through IIIF--just scanned files. This is the right time to create a more robust ingestion pipeline, and rearchitect that side of the app.

A front-end, user-upload interface is not really needed now, we can load these all via command line for the time being. This ticket is more about organizing the code base to accomplish what we need now while setting the foundation for future work.

This work should include moving all of the LOC-specific classes and logic from the loc_insurancemaps app and into the new core app, likely within a new importers module or something along those lines.