ExtractAPI: avoid unnecessary duplication of defs with primitive types

If a method's type contains a non-primitive value class then it has two
signatures: one before erasure and one after erasure. Before this
commit, we checked if this was the case using `isAnyValSubtype`, but
this is too crude since primitive value classes are also subtypes of
`AnyVal` but do not change signature after erasure.

This commit replaces `isAnyValSubtype` by `isDerivedValueClass` which
excludes primitive value classes.

In practice, for an empty class, this reduces the size of the output of
`DefaultShowAPI` from 65 lines to 25 lines.
Before:
https://gist.github.com/smarter/cf1d6fe58efda88d6ee6#file-old-api
After:
https://gist.github.com/smarter/cf1d6fe58efda88d6ee6#file-new-api

compile/interface/src/main/scala/xsbt/Compat.scala

@@ -107,8 +107,8 @@ abstract class Compat {
     }
     lazy val AnyValClass = global.rootMirror.getClassIfDefined("scala.AnyVal")
 
-    def isAnyValSubtype(sym: Symbol): Boolean = sym.isNonBottomSubClass(AnyValClass)
-
+    def isDerivedValueClass(sym: Symbol): Boolean =
+      sym.isNonBottomSubClass(AnyValClass) && !definitions.ScalaValueClasses.contains(sym)
   }
 
   object MacroExpansionOf {

compile/interface/src/main/scala/xsbt/ExtractAPI.scala

@@ -205,14 +205,14 @@ class ExtractAPI[GlobalType <: CallbackGlobal](val global: GlobalType,
 
       val hasValueClassAsParameter: Boolean = {
         import MirrorHelper._
-        s.asMethod.paramss.flatten map (_.info) exists (t => isAnyValSubtype(t.typeSymbol))
+        s.asMethod.paramss.flatten map (_.info) exists (t => isDerivedValueClass(t.typeSymbol))
       }
 
       def hasValueClassAsReturnType(tpe: Type): Boolean = tpe match {
         case PolyType(_, base) => hasValueClassAsReturnType(base)
         case MethodType(_, resultType) => hasValueClassAsReturnType(resultType)
         case Nullary(resultType) => hasValueClassAsReturnType(resultType)
-        case resultType => isAnyValSubtype(resultType.typeSymbol)
+        case resultType => isDerivedValueClass(resultType.typeSymbol)
       }
 
       val inspectPostErasure = hasValueClassAsParameter || hasValueClassAsReturnType(viewer(in).memberInfo(s))