Consider extracting FactoryBot cops to a separate gem

[bquorning]

See original suggestion in #1044 (comment), #1044 (comment) and #1044 (comment).

This was suggested because of challenges with the AllCops → RSpec/FactoryBot → Patterns configuration and our “file relevance detection” code.

[bquorning]

The underlying problem is fixed by #1063, which will be released as part of v2.0.