[cloud_firestore_odm] Have an error if the @Collection is in a different file from the model

[TarekkMA]

Moved from: firebase/flutterfire#11236

Related to firebase/flutterfire#11228

Currently, if the model & collection reference are in different files, we'll get a compilation error.
This is a known limitation, but we are lacking a nice error message to catch this early on. At the moment, folks would have to find it the hard way, by having a compilation error in the generated code and googling why that is the case.

[diegogarciar]

also related to #2