[nem-en] NamespaceTree.LookupType with -1

[vc]

Hi,

I think, use NamespaceTree.LookupType() with args_count == -1 is dangerou.

It's lead to error if exists ambiguity types (for example calss A {} and
class A[T] {}).

Maybe it is better if we remove this behavior from LookupType and
LookupExactType?

Vlad