Workaround bug in Scala runtime reflection on JDK 9

The underlying bug is tracked as https://github.com/scala/scala-dev/issues/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

12 files changed, 24 insertions, 0 deletions

diff --git a/project/BuildSettings.scala b/project/BuildSettings.scala
index 76cd888a2d..8456f91f86 100644
--- a/project/BuildSettings.scala
+++ b/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 */
diff --git a/project/GenerateAnyVals.scala b/project/GenerateAnyVals.scala
index 84454cb0ed..f349bfd16b 100644
--- a/project/GenerateAnyVals.scala
+++ b/project/GenerateAnyVals.scala
@@ -1,3 +1,5 @@
+package scala.build
+
 /** Code generation of the AnyVal types and their companions. */
 trait GenerateAnyValReps {
   self: GenerateAnyVals =>
diff --git a/project/JarJar.scala b/project/JarJar.scala
index 918060c9ee..3cb9e4cfff 100644
--- a/project/JarJar.scala
+++ b/project/JarJar.scala
@@ -1,3 +1,5 @@
+package scala.build
+
 import org.pantsbuild.jarjar
 import org.pantsbuild.jarjar._
 import org.pantsbuild.jarjar.util._
diff --git a/project/MiMa.scala b/project/MiMa.scala
index ceda8f5594..fb9bb175ab 100644
--- a/project/MiMa.scala
+++ b/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
diff --git a/project/Osgi.scala b/project/Osgi.scala
index 082fd91ed1..b05751958a 100644
--- a/project/Osgi.scala
+++ b/project/Osgi.scala
@@ -1,3 +1,5 @@
+package scala.build
+
 import aQute.bnd.osgi.Builder
 import aQute.bnd.osgi.Constants._
 import java.util.Properties
diff --git a/project/ParserUtil.scala b/project/ParserUtil.scala
index cdaf8831a5..bbd9129dbe 100644
--- a/project/ParserUtil.scala
+++ b/project/ParserUtil.scala
@@ -1,3 +1,5 @@
+package scala.build
+
 import sbt._
 import sbt.complete.Parser._
 import sbt.complete.Parsers._
diff --git a/project/PartestUtil.scala b/project/PartestUtil.scala
index ab7e62b3b4..f48ac2b693 100644
--- a/project/PartestUtil.scala
+++ b/project/PartestUtil.scala
@@ -1,3 +1,5 @@
+package scala.build
+
 import sbt._
 import sbt.complete._, Parser._, Parsers._
 
diff --git a/project/Quiet.scala b/project/Quiet.scala
index 0a186d8f28..8ae08ad5a6 100644
--- a/project/Quiet.scala
+++ b/project/Quiet.scala
@@ -1,3 +1,5 @@
+package scala.build
+
 import sbt._
 import Keys._
 
diff --git a/project/ScalaOptionParser.scala b/project/ScalaOptionParser.scala
index 27ed1f0e6f..0208921959 100644
--- a/project/ScalaOptionParser.scala
+++ b/project/ScalaOptionParser.scala
@@ -1,3 +1,5 @@
+package scala.build
+
 import ParserUtil._
 import sbt._
 import sbt.complete.Parser._
diff --git a/project/ScalaTool.scala b/project/ScalaTool.scala
index 98e18235c4..ace547c640 100644
--- a/project/ScalaTool.scala
+++ b/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
diff --git a/project/ScriptCommands.scala b/project/ScriptCommands.scala
index 8d5d09943a..f6b700f007 100644
--- a/project/ScriptCommands.scala
+++ b/project/ScriptCommands.scala
@@ -1,3 +1,5 @@
+package scala.build
+
 import sbt._
 import Keys._
 import BuildSettings.autoImport._
diff --git a/project/VersionUtil.scala b/project/VersionUtil.scala
index 7c4909697f..ebc2488345 100644
--- a/project/VersionUtil.scala
+++ b/project/VersionUtil.scala
@@ -1,3 +1,5 @@
+package scala.build
+
 import sbt._
 import Keys._
 import java.util.Properties