[JS IR] Support per-file mode and ES modules

js/js.translator/testData/box/esModules/jsModule/externalClassNameClash.kt

@@ -0,0 +1,62 @@
+// DONT_TARGET_EXACT_BACKEND: JS
+// ES_MODULES
+// FILE: a.kt
+@file:JsModule("./a.mjs")
+package a
+
+external class A {
+    fun foo(): String
+}
+
+external fun bar(): Int
+
+external val prop: Int
+
+// FILE: b.kt
+@file:JsModule("./b.mjs")
+package b
+
+external class A {
+    fun foo(): String
+}
+
+external fun bar(): Int
+
+external var prop: Int
+
+// FILE: main.kt
+
+import a.A as O
+import b.A as K
+
+fun box(): String {
+    if (a.bar() != 1) return "fail 1"
+    if (a.prop != 10) return "fail 2"
+    if (b.bar() != 2) return "fail 3"
+    if (b.prop != 20) return "fail 4"
+
+    return O().foo() + K().foo()
+}
+
+// FILE: a.mjs
+export function A() {}
+
+A.prototype.foo = function () {
+   return "O";
+};
+
+export function bar() { return 1; }
+
+export let prop = 10
+
+
+// FILE: b.mjs
+export function A() {}
+
+A.prototype.foo = function () {
+    return "K";
+};
+
+export function bar() { return 2; }
+
+export let prop = 20;