Overriding d7_file, d7_file_entity_item plugins requires source_name values not be changed

Thanks to help from fjgarlin on DrupalSlack the problem was straightforward - the "source_module" on the custom plugins should have been set to "file" for the d7_file plugin, and "file_entity" for the d7_file_entity_item plugin.

I thought this could be a learning exercise, so here's some documentation.

Marking this as RTBC as I ran into the same issue a few months ago and the solution was exactly what's documented here.