Workaround bug in Scala runtime reflection on JDK 9

The underlying bug is tracked as scala/scala-dev#304
and blocks our SBT starting on JDK 9.

This commit avoids using the empty package in our build definition.

After this change, I needed to manually clean the class files from the
build definition as follows, which might indicate a bug in SBT.

   $ sbt ...
   /Users/jz/code/scala-java9-ci/build.sbt:0: warning: imported `BuildSettings' is permanently hidden by definition of object BuildSettings
   import ..., _root_.scala.build.BuildSettings, ...
   ^C

   % rm -rf project/target/scala-2.10/sbt-0.13/classes/

   % sbt # okay second time

build.sbt

@@ -32,6 +32,7 @@
  *   - to modularize the Scala compiler or library further
  */
 
+import scala.build._
 import VersionUtil._
 
 // Scala dependencies:

project/BuildSettings.scala

@@ -1,3 +1,5 @@
+package scala.build
+
 import sbt._
 
 /** This object defines keys that should be visible with an unqualified name in all .sbt files and the command line */

project/GenerateAnyVals.scala

@@ -1,3 +1,5 @@
+package scala.build
+
 /** Code generation of the AnyVal types and their companions. */
 trait GenerateAnyValReps {
   self: GenerateAnyVals =>

project/JarJar.scala

@@ -1,3 +1,5 @@
+package scala.build
+
 import org.pantsbuild.jarjar
 import org.pantsbuild.jarjar._
 import org.pantsbuild.jarjar.util._

project/MiMa.scala

@@ -1,3 +1,5 @@
+package scala.build
+
 // It would be nice to use sbt-mima-plugin here, but the plugin is missing
 // at least two features we need:
 // * ability to run MiMa twice, swapping `curr` and `prev`, to detect

project/Osgi.scala

@@ -1,3 +1,5 @@
+package scala.build
+
 import aQute.bnd.osgi.Builder
 import aQute.bnd.osgi.Constants._
 import java.util.Properties

project/ParserUtil.scala

@@ -1,3 +1,5 @@
+package scala.build
+
 import sbt._
 import sbt.complete.Parser._
 import sbt.complete.Parsers._

project/PartestUtil.scala

@@ -1,3 +1,5 @@
+package scala.build
+
 import sbt._
 import sbt.complete._, Parser._, Parsers._
 

project/Quiet.scala

@@ -1,3 +1,5 @@
+package scala.build
+
 import sbt._
 import Keys._
 

project/ScalaOptionParser.scala

@@ -1,3 +1,5 @@
+package scala.build
+
 import ParserUtil._
 import sbt._
 import sbt.complete.Parser._

project/ScalaTool.scala

@@ -1,3 +1,5 @@
+package scala.build
+
 import sbt._
 import org.apache.commons.lang3.SystemUtils
 import org.apache.commons.lang3.StringUtils.replaceEach

project/ScriptCommands.scala

@@ -1,3 +1,5 @@
+package scala.build
+
 import sbt._
 import Keys._
 import BuildSettings.autoImport._

project/VersionUtil.scala

@@ -1,3 +1,5 @@
+package scala.build
+
 import sbt._
 import Keys._
 import java.util.Properties