Fix engine requires for msfconsole

MSP-11412

**NB: This story should not land until the following PRs have closed. They should be closed IN THE FOLLOWING ORDER:**

- [x] rapid7/metasploit-concern#9
- [x] rapid7/metasploit-model#29
- [x] rapid7/metasploit_data_models#81
- [x] rapid7/metasploit-credential#75

## Verification steps

- [x] `rake spec`
- [x] `rake features`
- [x] `msfconsole` boots without error
- [x] `bundle install --without db` and try the above steps again

## Post-merge steps
- [x] `bundle`
- [x] `rake spec`
- [x] push master