Shorten references: Do not lookup packages when searching for conflicting declarations

Minor: use JetTreeVisitor

idea/testData/shortenRefs/type/ClassSameNameAsPackage.dependency.kt

@@ -0,0 +1,3 @@
+package same
+
+class same() {}

idea/testData/shortenRefs/type/ClassSameNameAsPackage.kt

@@ -0,0 +1,2 @@
+fun f(s: <selection>same.same</selection>) {
+}

idea/testData/shortenRefs/type/ClassSameNameAsPackage.kt.after

@@ -0,0 +1,4 @@
+import same.same
+
+fun f(s: same) {
+}