Change importing rules for objects:

Allow importing classes from object, prohibit import-on-demand from objects
It's unclear what import-on-demand from object should mean so we prohibit it for now

compiler/testData/diagnostics/tests/imports/Imports.kt

@@ -70,7 +70,7 @@ fun bar() {}
 //FILE:c.kt
 package c
 
-import c.<!CANNOT_IMPORT_FROM_ELEMENT!>C<!>.*
+import c.<!CANNOT_IMPORT_ON_DEMAND_FROM_SINGLETON!>C<!>.*
 
 object C {
     fun f() {