Stub generation infrastructure (except for metadata generation) for KAPT+K2

This commit includes the basic Java stubs generation infrastructure and
the corresponding tests. The main entry point is called
Kapt4StubGenerator. Calls to it from production code will be added in a
separate commit.

 #KT-51982

plugins/kapt3/kapt3-compiler/testData/converter/multifileClassDefaultPackage.txt

@@ -0,0 +1,40 @@
+@kotlin.Metadata()
+public final class AnotherRootClass {
+
+    public AnotherRootClass() {
+        super();
+    }
+}
+
+////////////////////
+
+
+@kotlin.Metadata()
+public final class RootClass {
+
+    public RootClass() {
+        super();
+    }
+}
+
+////////////////////
+
+package test;
+
+@kotlin.Metadata()
+public final class M1 {
+
+    public M1() {
+        super();
+    }
+
+    @org.jetbrains.annotations.Nullable()
+    public static final RootClass foo() {
+        return null;
+    }
+
+    @org.jetbrains.annotations.Nullable()
+    public static final AnotherRootClass bar() {
+        return null;
+    }
+}