Define populate virtuals on document arrays on the document array, not top-level document

[vkarpov15]

Re: #8198 , it looks like if you do:

const FooSchema = new Schema({
  name: String,
  children: [{
    barId: { type: Schema.Types.ObjectId, ref: 'Bar' },
    quantity: Number,
  }],
}, {
  toJSON: { virtuals: true },
  toObject: { virtuals: true },
});
FooSchema.virtual('children.bar', {
  ref: 'Bar',
  localField: 'children.barId',
  foreignField: '_id',
  justOne: true,
});

children.bar is a top-level virtual and populated virtuals live on the top-level document, leading to weird issues if you access the virtual using children[0].bar as opposed to get('children.0.bar').