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/overlappingFuns.kt

@@ -16,7 +16,7 @@ private fun overlapping() = "oops #1"
 @file:[JvmName("MultifileClass") JvmMultifileClass]
 package a
 
-fun overlapping() = "OK"
+private fun overlapping() = "OK"
 
 fun ok() = overlapping()