This one is a bit more involved. Of note is the fact that it at present
only uses local flow when determining the origin of some value (whereas
the points-to version used global flow). It may be desirable to rewrite
this query to use global data-flow, but this should be done with some
care (as using "all unhashable objects" as the set of sources is
somewhat iffy with respect to performance). For that reason, I'm
sticking to mostly local flow (except for well behaved things like types
and built-ins).
Uses the new `DuckTyping` module to handle recognising whether a class
is a container or not. Only trivial test changes (one version uses
"class", the other "Class").
Note that the ported query has no understanding of built-in classes. At
some point we'll likely want to replace `hasUnresolvedBase` (which will
hold for any class that extends a built-in) with something that's aware
of the built-in classes.
I'm beginning to realise why I didn't do the `toString` overriding way
back when. Thankfully, now that all of our tests are in the same place,
this is actually not a terrible ordeal.
