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.

idea/testData/diagnosticMessage/conflictingOverloadsClass1.txt

@@ -0,0 +1,2 @@
+<!-- conflictingOverloadsClass1 -->
+Conflicting overloads: public final fun lol(x: Int): Int defined in conflictingOverloads, public final fun lol(y: Int): Int defined in conflictingOverloads