KT-13961 REDECLARATION not reported on private-in-file 'foo' vs public 'foo' in different file

Private-in-file declarations conflict with public overload-equivalent declarations
in other files in the same package.

Move functions for grouping possible redeclarations to OverloadResolver
(since they are used only there).

Refactor redeclarations / conflicting overloads reporting.

compiler/testData/diagnostics/tests/redeclarations/ScriptAndClassConflict.kt

@@ -1,7 +1,7 @@
 // FILE: f1.kt
 package test
 
-class <!REDECLARATION!>A<!>
+class <!PACKAGE_OR_CLASSIFIER_REDECLARATION!>A<!>
 class F1
 
 // FILE: A.kts