KT-2752: fix JsName with explicit use target. Add some tests for JsName with use targets

js/js.translator/testData/box/multiFile/packageAndMangledMethodDoNotClash.kt

@@ -3,10 +3,10 @@ package foo
 
 fun bar(x: Int) = x
 
-fun box(): Boolean {
+fun box(): String {
     assertEquals(23, bar(23))
     assertEquals(42, foo.bar.x)
-    return true
+    return "OK"
 }
 
 // FILE: foobar.kt

js/js.translator/testData/box/multiFile/packageAndPrivateDeclarationDoNotClash.kt

@@ -5,12 +5,12 @@ private fun bar() = 23
 
 private val bar = 42
 
-fun box(): Boolean {
+fun box(): String {
     assertEquals(23, bar())
     assertEquals(42, bar)
     assertEquals(32, foo.bar.x)
 
-    return true
+    return "OK"
 }
 
 // FILE: foobar.kt