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:*

* rapid7/metasploit-concern#9
* rapid7/metasploit-model#29
* rapid7/metasploit_data_models#81
* rapid7/metasploit-credential#75

## Verification steps

- [ ] `rake spec`
- [ ] `rake features`
- [ ] `msfconsole` boots without error
- [ ] `bundle install --without db` and try the above steps again