Refactor extract_transform_and_load_covid_data

[lucasmbrown]

What started out as a small method has grown into a behemoth that's pretty hard to follow. (And I'm the one mostly to blame for it growing that way, so no shame on anyone else!)

Can we take a thoughtful scalpel to this method and refactor it into smaller, more comprehensible pieces? That might make updates and testing easier in the future.