There might be a silent bug in the Python module which directly reads
from this type_set field; in particular, it does so when reading all
controlling type variables, which all seem to be free variables (i.e. no
parent typevar). So imitate this behavior here, until we're sure there
are no other meta generators that rely on this.
Fixes two issues:
- name of derived type variables wasn't correct. (This is used during
code generation.)
- the TypeVar::derived() function wasn't creating the correct type set
(and would instead propagate the parent's one).
