Disambiguate types when they have the same name and need cast between each other.

test/test_suite/cast/implicit_cast_same_name.c3

@@ -0,0 +1,15 @@
+module foo::a;
+struct Lexer { int a; }
+fn void foo(Lexer *lex) {}
+
+module foo::b;
+struct Lexer { int a; }
+fn void foo(Lexer *lex) {}
+
+module foo;
+
+fn void bar()
+{
+  a::Lexer l;
+  b::foo(&l); // #error: Implicitly casting 'foo::a::Lexer*' to 'foo::b::Lexer*' is not permitted, but you may do an explicit cast by placing '(foo::b::Lexer*)' before the expression
+}