Add extra classpath to the metabuild

When sbt was entered through xMain.run and the classloaders do not have
the expected format, sbt recreates the classloaders for itself.
Unfortunately the extra classpath was not added to the classloader. This
caused project/extra to fail if it was entered from RunFromSourceMain
rather than with the launcher.

main/src/main/java/sbt/internal/MetaBuildLoader.java

@@ -11,6 +11,8 @@ import java.io.File;
 import java.io.IOException;
 import java.net.URL;
 import java.net.URLClassLoader;
+import java.util.LinkedHashSet;
+import java.util.Set;
 import java.util.regex.Pattern;
 import xsbti.AppProvider;
 import xsbti.ScalaProvider;
@@ -61,19 +63,27 @@ public final class MetaBuildLoader extends URLClassLoader {
     final Pattern pattern = Pattern.compile("test-interface-[0-9.]+\\.jar");
     final File[] cp = appProvider.mainClasspath();
     final URL[] interfaceURL = new URL[1];
-    final URL[] rest = new URL[cp.length - 1];
+    final File[] extra =
+        appProvider.id().classpathExtra() == null ? new File[0] : appProvider.id().classpathExtra();
+    final Set<File> bottomClasspath = new LinkedHashSet<>();
 
     {
-      int i = 0;
+      for (final File file : cp) {
-      int j = 0; // index into rest
-      while (i < cp.length) {
-        final File file = cp[i];
         if (pattern.matcher(file.getName()).find()) {
           interfaceURL[0] = file.toURI().toURL();
         } else {
-          rest[j] = file.toURI().toURL();
+          bottomClasspath.add(file);
-          j += 1;
         }
+      }
+      for (final File file : extra) {
+        bottomClasspath.add(file);
+      }
+    }
+    final URL[] rest = new URL[bottomClasspath.size()];
+    {
+      int i = 0;
+      for (final File file : bottomClasspath) {
+        rest[i] = file.toURI().toURL();
         i += 1;
       }
     }