[K/JS] Implement file merging for clashed file during per-file compilation

js/js.translator/testData/box/esModules/export/fileNameClash.kt

@@ -0,0 +1,35 @@
+// DONT_TARGET_EXACT_BACKEND: JS
+// ES_MODULES
+
+// MODULE: name_clash
+// FILE: test1/lib.kt
+package foo
+
+@JsExport
+open class A<T> {
+    open fun foo(value: T): T = value
+}
+
+// FILE: test2/lib.kt
+package foo
+
+@JsExport
+class B: A<String>() {
+    override fun foo(value: String): String = value
+}
+
+// FILE: entry.mjs
+// ENTRY_ES_MODULE
+import { A, B } from "./fileNameClash-name_clash_v5.mjs";
+
+export function box() {
+    var a = new A()
+    var aFoo = a.foo("ok")
+    if (aFoo != "ok") return "fail 1"
+
+    var b = new B()
+    var bFoo = b.foo("ok")
+    if (bFoo != "ok") return "fail 2"
+
+    return "OK"
+}