KT-36860 Collect extensions from object on first completion

- This should not affect the performance of the completion, since all
object extensions are collected on the last step, when all main variants
are already collected
- Add more tests
- Also, disable completion of extensions from objects as callable
references (^KT-37395 Fixed)
- ^KT-36860 Fixed

idea/idea-completion/testData/basic/common/extensionMethodInObject/ObjectExplicitReceiver.kt

@@ -9,6 +9,5 @@ fun usage(t: T) {
     t.foo<caret>
 }
 
-// INVOCATION_COUNT: 2
 // EXIST: { lookupString: "fooExtension", itemText: "fooExtension" }
 // EXIST: { lookupString: "fooProperty", itemText: "fooProperty" }