Refactor GetMember/GetMemberType to reuse _get_members_dict
Extract the member-building logic from _eval_members_impl into
_get_members_dict (and _get_typeddict_members_dict) returning a
dict[str, Type]. Members/Attrs/GetMember/GetMemberType all now
share this common helper. GetMemberType falls back to direct
attribute lookup for stub types like Member itself.
