ShortenReferences: cleanup code & add tests for property-function conflicts

idea/idea-completion/testData/handlers/multifile/PropertyFunctionConflict-1.kt

@@ -0,0 +1,7 @@
+package a
+
+class Goo {
+    fun x() {
+        foo<caret>
+    }
+}

idea/idea-completion/testData/handlers/multifile/PropertyFunctionConflict-2.kt

@@ -0,0 +1,5 @@
+package a.b
+
+fun foo(i: Int) {}
+internal val Foo.foo: Int get() = 123
+class Foo {}

idea/idea-completion/testData/handlers/multifile/PropertyFunctionConflict.kt.after

@@ -0,0 +1,9 @@
+package a
+
+import a.b.foo
+
+class Goo {
+    fun x() {
+        foo()
+    }
+}