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/codegen/box/multifileClasses/optimized/overlappingVals.kt

@@ -4,7 +4,7 @@
 
 import a.*
 
-fun box(): String = overlapping
+fun box(): String = ok()
 
 // FILE: part1.kt
 @file:[JvmName("MultifileClass") JvmMultifileClass]
@@ -16,7 +16,9 @@ private val overlapping = run { "oops #1" }
 @file:[JvmName("MultifileClass") JvmMultifileClass]
 package a
 
-val overlapping = run { "OK" }
+private val overlapping = run { "OK" }
+
+fun ok() = overlapping
 
 // FILE: part3.kt
 @file:[JvmName("MultifileClass") JvmMultifileClass]