New J2K: do not find import usages in AST building phase

It is a rather heavy operation and it increases conversion time
Related to #KT-31848

nj2k/testData/newJ2k/assignmentExpression/nullability-settings.kt

@@ -1,6 +1,8 @@
 // !forceNotNullTypes: false
 // !specifyLocalVariableTypeByDefault: true
 
+import java.util.HashSet
+
 internal class Foo {
     fun foo(o: HashSet<*>) {
         val o2: HashSet<*> = o