fix(schema): make Schema.prototype.clone() avoid creating different copies of subdocuments and single nested paths underneath single nested paths

[vkarpov15]

Re: #13626

Summary

#13626 is one of those issues where doing the "5 whys" exercise takes you down a very deep rabbit hole. There are a few issues that came together to form the issue reported in #13626. The one addressed in this PR is that Schema.prototype.clone() unintentionally creates duplicate constructors for subdocuments and document arrays that are nested underneath a subdocument path.

new Schema({
  docArr1: [{ name: String }] // unaffected
  subdoc1: new Schema({
    docArr2: [{ name: String }], // affected
    subdoc2: new Schema({ name: String }) // affected
  });
}).clone();

The issue is that Schema.prototype.clone() first uses utils.clone() to clone the schema's paths, and then clone the single nested paths:

s.paths = utils.clone(this.paths);
s.singleNestedPaths = utils.clone(this.singleNestedPaths);

clone() calls SchemaType.prototype.clone(), on each schema type it finds. This means that, after cloning singleNestedPaths, paths['subdoc1'].schema.paths['subdoc2'] will not be the same as singleNestedPaths['subdoc1.subdoc2']. They will be distinct clones of the original schema's singleNestedPaths['subdoc1.subdoc2'].

Fix is to re-assemble singleNestedPaths from the cloned schemas' paths.

Examples

[Uzlopak]

LGTM