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/errors.txt

@@ -1,4 +1,3 @@
-// -- Module: <api> --
 package
 
 package api {
@@ -11,7 +10,7 @@ package api {
         public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
     }
 
-    @kotlin.Experimental(changesMayBreak = {Impact.COMPILATION}, level = Level.ERROR) public final annotation class E : kotlin.Annotation {
+    @kotlin.Experimental public final annotation class E : kotlin.Annotation {
         public constructor E()
         public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
         public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
@@ -19,13 +18,6 @@ package api {
     }
 }
 
-
-// -- Module: <usage> --
-package
-
-package api {
-}
-
 package usage {
     public fun test(/*0*/ b: api.Base): kotlin.Unit