Filter out overridden candidates before looking for most specific

This filtering will remove signature duplicates that prevent findMaximallySpecific() from finding best candidate.

compiler/testData/diagnostics/tests/multimodule/varargConflict.kt

@@ -0,0 +1,26 @@
+// !DIAGNOSTICS: -UNUSED_VARIABLE -UNUSED_PARAMETER
+// MODULE: m1
+// FILE: a.kt
+
+package p
+
+public fun foo(a: Int) {}
+public fun foo(vararg values: Int) {}
+
+// MODULE: m2
+// FILE: b.kt
+
+package p
+
+public fun foo(a: Int) {}
+public fun foo(vararg values: Int) {}
+
+// MODULE: m3(m1, m2)
+// FILE: c.kt
+package m
+
+import p.foo
+
+fun main() {
+    foo(12)
+}