fix(schema): recursively copy schemas from different modules when calling new Schema()

[vkarpov15]

Fix #13275

Summary

Right now, you can call new Schema() on a schema from a different copy of the Mongoose module to "copy" the schema over to use the local Mongoose module's schema class. But that doesn't apply to child schemas. This PR makes it so that child schemas also merge correctly.

Examples

[AbdelrahmanHafez]

Can we have a test-case to prevent a future regression?

[vkarpov15]

Adding a test case for this is pretty difficult, and IMO not really worth the effort. We'd have to do some pretty sophisticated trickery to run a script that loads two copies of the Mongoose module without polluting require() in the actual test code.

[IslandRhythms]

LGTM