FIR: Reimplement conflicts checker to detect conflicts in different files

also pass correct ScopeSession to checkers
fixes some IC tests

compiler/tests-spec/testData/diagnostics/notLinked/annotations/type-annotations/neg/3.fir.kt

@@ -5,8 +5,6 @@
 annotation class Ann(val x: Int)
 
 // TESTCASE NUMBER: 1
-class Inv<T>
-
 fun foo(i: Inv<@Ann(unresolved_reference) String>) {}
 
 // TESTCASE NUMBER: 2

compiler/tests-spec/testData/diagnostics/notLinked/annotations/type-annotations/neg/4.fir.kt

@@ -2,6 +2,4 @@
 @Target(AnnotationTarget.TYPE)
 annotation class Ann(val x: Int)
 
-class Inv<T>
-
 fun case_1(): Inv<@Ann(unresolved_reference) String> = TODO()