go/types, types2: print instantiated types with package qualification in type cycles spanning external packages

[griesemer]

This is a reminder issue to fix the printing of external instantiated packages (as reported with cycle errors) that contribute to invalid type cycles. Add corresponding tests.

See Checker.validType implementation, when we call check.cycleError(path[i:]) for a cycle that spans important instantiated types.

cc: @findleyr