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/scripts.kt

@@ -1,17 +1,15 @@
 // !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)
 annotation class ExperimentalAPI
 
 @ExperimentalAPI
 fun function(): String = ""
 
-// MODULE: usage1(api)
 // FILE: usage-propagate.kts
 
 import api.*
@@ -24,7 +22,6 @@ fun use() {
 <!EXPERIMENTAL_API_USAGE!>function<!>()
 <!EXPERIMENTAL_API_USAGE!>use<!>()
 
-// MODULE: usage2(api)
 // FILE: usage-use.kts
 
 @file:UseExperimental(ExperimentalAPI::class)
@@ -34,10 +31,9 @@ fun use() {
     function()
 }
 
-<!EXPERIMENTAL_API_USAGE!>function<!>()
+function()
 use()
 
-// MODULE: usage3(api)
 // FILE: usage-none.kts
 
 import api.*