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/constVal.kt

@@ -1,10 +1,9 @@
 // !API_VERSION: 1.3
-// MODULE: api
 // FILE: api.kt
 
 package api
 
-@Experimental(Experimental.Level.WARNING, [Experimental.Impact.COMPILATION])
+@Experimental(Experimental.Level.WARNING)
 @Target(AnnotationTarget.PROPERTY, AnnotationTarget.FUNCTION)
 annotation class ExperimentalAPI
 
@@ -13,7 +12,6 @@ const val MEANING = 42
 
 annotation class Anno(val value: Int)
 
-// MODULE: usage1(api)
 // FILE: usage-propagate.kt
 
 package usage1
@@ -24,7 +22,6 @@ import api.*
 @Anno(MEANING)
 fun usage() {}
 
-// MODULE: usage2(api)
 // FILE: usage-use.kt
 
 @file:UseExperimental(ExperimentalAPI::class)
@@ -32,10 +29,10 @@ package usage2
 
 import api.*
 
+// TODO: there should be no warning here
 @Anno(<!EXPERIMENTAL_API_USAGE!>MEANING<!>)
 fun usage() {}
 
-// MODULE: usage3(api)
 // FILE: usage-none.kt
 
 package usage3