[FIR] Implement CONFLICTING_IMPORT diagnostic

compiler/testData/diagnostics/tests/imports/NestedClassClash.fir.kt

@@ -1,31 +0,0 @@
-// !DIAGNOSTICS: -UNUSED_PARAMETER
-// FILE: a.kt
-package a
-
-class A {
-    class B
-}
-
-// FILE: b.kt
-package a
-
-class D {
-    class B
-}
-
-// FILE: c.kt
-import a.A.B
-import a.D.B
-
-fun test(b: <!UNRESOLVED_REFERENCE!>B<!>) {
-    <!UNRESOLVED_REFERENCE!>B<!>()
-}
-
-// FILE: d.kt
-import a.A.*
-import a.D.*
-
-// todo ambiguvity here
-fun test2(b: <!UNRESOLVED_REFERENCE!>B<!>) {
-    <!UNRESOLVED_REFERENCE!>B<!>()
-}