Generating .rbi file for an ActiveModel model

[xcskier56]

Describe the bug:
I have a non-database backed model that is including ActiveModel::Model. When I run either bundle exec rails_rbi:all and/or bundle exec rails_rbi:models, no .rbi file is generated for my class.

Steps to reproduce:

1. Create a non-ActiveRecord class in your models folder and have it include ActiveModel::Model
2. Generate rbi files using bundle exec rails_rbi:all
3. Note that no new rbi file is generated

Expected behavior:
It would generate a new rbi file

Versions:

• Ruby: 2.6.6p146
• Rails: 6.0.3.2
• Sorbet: 0.5.5909
• Sorbet-Rails: 0.7.1