Remove dependency on populating top-level document paths

[vkarpov15]

Re: #8273 and #8247, it looks like we sometimes depend on setting populated() in the top-level document (

mongoose/lib/document.js

Lines 1151 to 1159 in dfde779

	if (this.ownerDocument) {
	popOpts = { [populateModelSymbol]: val[0].constructor };
	this.ownerDocument().populated(this.$__fullPath(path),
	val.map(function(v) { return v._id; }), popOpts);
	} else {
	popOpts = { [populateModelSymbol]: val[0].constructor };
	this.populated(path, val.map(function(v) { return v._id; }), popOpts);
	}
	didPopulate = true;

) and sometimes we don't (

mongoose/lib/document.js

Lines 1137 to 1139 in dfde779

	this.populated(path, val._id, { [populateModelSymbol]: val.constructor });
	didPopulate = true;
	}

) . The latter is the better approach because once you have populated paths within a document array, or populated paths within a single nested doc within an array, it becomes increasingly difficult to figure out populated values for the subdocument you're looking at.