Move generate_workspace to src/tools

Seems like a better location for it (easier for people to run than
src/main/java/com/google/devtools/build etc).

--
MOS_MIGRATED_REVID=102354250
diff --git a/src/BUILD b/src/BUILD
index e9c0f3d..3d672ad 100644
--- a/src/BUILD
+++ b/src/BUILD
@@ -10,8 +10,8 @@
         "//conditions:default": ["//src/main/native:libunix.so"],
     }),
     visibility = [
-        "//src/main/java/com/google/devtools/build/workspace:__pkg__",
         "//src/test/java:__pkg__",
+        "//src/tools/generate_workspace:__pkg__",
     ],
 )