BREAKING CHANGE: call virtual ref function with subdoc, not top-level doc

[vkarpov15]

Fix #12363
Fix #12440

Summary

The primary issue from #12363 and #12440 is that right now we call ref() functions on virtuals with the top-level document rather than the subdocument, even if the virtual is defined on the subdocument. For example, in Mongoose 8, with the following setup, ref() would see this as an instance of ParentSchema, not the b subdocument.

        const ASchema = new Schema({
          name: String
        });

        const BSchema = new Schema({
          referencedModel: String,
          aId: ObjectId
        });

        BSchema.virtual('a', {
          ref: function() {
            return this.referencedModel;
          },
          localField: 'aId',
          foreignField: '_id',
          justOne: true
        });

        const ParentSchema = new Schema({
          b: BSchema
        });

        const A1 = db.model('Test1', ASchema);
        const A2 = db.model('Test2', ASchema);
        const Parent = db.model('Parent', ParentSchema);

With this PR, we will call ref() with the subdocument. This is more consistent with how things work in conventional populate: ref() would get called with the subdoc in virtual populate, see #8469 and this test.

Examples

If you're using ref() as a function, it is easier to get the top-level document from the subdocument (using subdoc.ownerDocument()) than it is to get the correct subdocument from the top-level document. Plus, calling ref() with the subdocument helps make schemas easier to reuse.

[hasezoey]

LGTM, should a migrate entry be added in this PR itself, or later?

[vkarpov15]

Later, since we don't have a migrating_to_9 doc yet