JS: refactor flat declaration generator. Add more tests on inter-module dependencies

js/js.translator/testData/box/crossModuleRef/inheritance.kt

@@ -0,0 +1,25 @@
+// MODULE: lib
+// FILE: lib.kt
+
+package lib
+
+open class A {
+    fun foo() = 23
+}
+
+// MODULE: main(lib)
+// FILE: main.kt
+
+package main
+
+import lib.A
+
+class B : A() {
+    fun bar() = foo() + 1
+}
+
+fun box(): String {
+    val result = B().bar()
+    if (result != 24) return "fail: $result"
+    return "OK"
+}