KT-9547: private package member can conflict only with the members

declared in the same file.
Cleanup OverloadUtil stuff.
Update testData.

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

@@ -1,9 +1,9 @@
 // FILE: a.kt
-val <!REDECLARATION, REDECLARATION!>a<!> : Int = 1
+val <!REDECLARATION!>a<!> : Int = 1
 <!CONFLICTING_OVERLOADS!>fun f()<!> {
 }
 
 // FILE: b.kt
-val <!REDECLARATION, REDECLARATION!>a<!> : Int = 1
+val <!REDECLARATION!>a<!> : Int = 1
 <!CONFLICTING_OVERLOADS!>fun f()<!> {
 }