Make stdlib work with -Xmultifile-package-facades.

Fixed wrong owner mapping in presence of -Xmultifile-package-facades.
Fixed backing field mapping issue.
Added more tests.

compiler/testData/codegen/boxMultifileClasses/calls/varFromOtherPackage.1.kt

@@ -0,0 +1,6 @@
+import a.OK
+
+fun box(): String {
+    OK = "OK"
+    return OK
+}