Run classifier usage checkers after all imports are resolved

Otherwise the diagnostic was not reported on an unused import of a deprecated
class in diagnostic tests, see the test case

compiler/testData/diagnostics/tests/deprecated/unusedImport.kt

@@ -0,0 +1,10 @@
+// FILE: A.kt
+
+package test
+
+@Deprecated("A")
+interface A
+
+// FILE: B.kt
+
+import test.<!DEPRECATION!>A<!>