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.

jps-plugin/testData/incremental/classHierarchyAffected/secondaryConstructorAdded/build.log

@@ -9,7 +9,7 @@ End of files
 Exit code: ABORT
 ------------------------------------------
 COMPILATION FAILED
-'public constructor A(x: String)' conflicts with another declaration in package '<root>'
+Conflicting overloads: public constructor A(x: String) defined in A, public fun A(x: String): A defined in root package
 
 ================ Step #2 =================
 
@@ -41,4 +41,4 @@ Compiling files:
   src/useA.kt
 End of files
 Exit code: OK
-------------------------------------------
+------------------------------------------