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/DetectPropertiesMultipleFiles/I.java

@@ -0,0 +1,21 @@
+package test;
+
+public interface I {
+    int getSomething1();
+
+    int getSomething2();
+
+    int getSomething3();
+
+    void setSomething3(int value);
+
+    int getSomething4();
+
+    void setSomething4(int value);
+
+    int getSomething5();
+
+    void setSomething5(int value);
+
+    void setSomething6(int value);
+}