(PUP-659) Change find class/define API by removing unused args

This removes the parameters for an array of namespaces to search
as well as the options (where it was possible to pass an assume_fqname
option with special meaning).

No the api is much simpler - lookup the name that is wanted; no more, no
less. It should be given in the correct form (downcased), but it may
start with '::'.