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/valFromOtherPackage.1.kt

@@ -0,0 +1,3 @@
+import a.OK
+
+fun box(): String = OK

compiler/testData/codegen/boxMultifileClasses/calls/valFromOtherPackage.2.kt

@@ -0,0 +1,4 @@
+@file:[JvmName("MultifileClass") JvmMultifileClass]
+package a
+
+val OK: String = "OK"

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

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

compiler/testData/codegen/boxMultifileClasses/calls/varFromOtherPackage.2.kt

@@ -0,0 +1,4 @@
+@file:[JvmName("MultifileClass") JvmMultifileClass]
+package a
+
+var OK: String = "Hmmm?"