Support lookup in aggregation pipelines with nested references

[alcaeus]

Feature Request

Q	A
New Feature	yes
RFC	no
BC Break	no

Summary

Based on documentation changes in #1934. Looking up a reference nested in an embedded document is not possible, since we only check hasReference($from) in the lookup stage. This should follow the query builder logic where ->field('foo.bar')->references($document) is valid and understood.