PS: Better usability when working with nested classes

[MathiasVP]

@chanel-y mentioned that the following never had any results:

from API::Node n
where
  n = API::getTopLevelMember("system")
          .getMember("security")
          .getMember("cryptography")
select n, "..."

whereas you did have a result with:

from API::Node n
where
  n = API::getTopLevelMember("system.security.cryptography")
select n, "..."

This was somewhat confusing, and I wasn't quite happy with that. So this PR makes it possible to do the former, while also keeping the latter.

While here, I also did some cleanup in the API graph libraries and added some dataflow predicates around models-as-data that i had forgotten to include 🙈