New J2K: make post-processing aware of other files which are being converted

Before post-processing was able to handle only one converting file at once
So, some conversions (like (field, getter, setter) to Kotlin property)
was not able to work when converting class hierarchy was split into
multiple files.
Also, inferring nullability for a set of files was broken

#KT-19569 fixed
#KT-34266 fixed
#KT-32518 fixed

nj2k/testData/multiFile/InterfaceWithGetterInOtherFile/IUser.java

@@ -0,0 +1,5 @@
+package test;
+
+public interface IUser {
+    String getName();
+}

nj2k/testData/multiFile/InterfaceWithGetterInOtherFile/IUser.kt

@@ -0,0 +1,5 @@
+package test
+
+interface IUser {
+    val name: String
+}

nj2k/testData/multiFile/InterfaceWithGetterInOtherFile/UserImpl.java

@@ -0,0 +1,14 @@
+package test;
+
+public class UserImpl implements IUser {
+    private final String name;
+
+    public UserImpl(String name) {
+        this.name = name;
+    }
+
+    @Override
+    public String getName() {
+        return name;
+    }
+}

nj2k/testData/multiFile/InterfaceWithGetterInOtherFile/UserImpl.kt

@@ -0,0 +1,3 @@
+package test
+
+class UserImpl(override val name: String) : IUser