Drop Experimental.Impact, simplify related code and tests

See https://github.com/Kotlin/KEEP/issues/95#issuecomment-383889404

Drop Experimental.changesMayBreak, Experimental.Impact, the concept of
signature/body usage, same module exemption. Make the majority of tests
single-module because there is now no difference in the checker between
usages from the same module or from another module

compiler/testData/diagnostics/testsWithStdLib/experimental/topLevel.kt

@@ -1,11 +1,10 @@
 // !API_VERSION: 1.3
 // !DIAGNOSTICS: -UNUSED_VARIABLE
-// MODULE: api
 // FILE: api.kt
 
 package api
 
-@Experimental(Experimental.Level.WARNING, [Experimental.Impact.COMPILATION])
+@Experimental(Experimental.Level.WARNING)
 @Target(AnnotationTarget.TYPE, AnnotationTarget.CLASS, AnnotationTarget.FUNCTION, AnnotationTarget.PROPERTY, AnnotationTarget.TYPEALIAS,
         AnnotationTarget.VALUE_PARAMETER)
 annotation class ExperimentalAPI
@@ -19,7 +18,6 @@ val property: String = ""
 @ExperimentalAPI
 typealias Typealias = String
 
-// MODULE: usage1(api)
 // FILE: usage-propagate.kt
 
 package usage1
@@ -42,7 +40,6 @@ class Use {
     }
 }
 
-// MODULE: usage2(api)
 // FILE: usage-use.kt
 
 package usage2
@@ -67,7 +64,6 @@ class Use {
     }
 }
 
-// MODULE: usage3(api)
 // FILE: usage-none.kt
 
 package usage3