Allow BUILD files directly under the build root

This makes the empty package name legal (//:foo). If the empty package is used,
this symlinks everything under the build root to the exec root.  This includes
directories.

--
MOS_MIGRATED_REVID=87960882
diff --git a/docs/build-ref.html b/docs/build-ref.html
index fe808ac..315603f 100644
--- a/docs/build-ref.html
+++ b/docs/build-ref.html
@@ -546,6 +546,11 @@
   '<code>_</code>', and '<code>/</code>'.
 </p>
 
+<p>
+  The empty string is also a valid package name (e.g., //:foo). Generally
+  projects should attempt to use more descriptively named packages, though.
+</p>
+
 
 <p>
   Package names may not contain the substring <code>//</code>, nor