Use java_binary to generate the source jar

The jar produced by java_source rule was not ending up at the root of
the buck output folder. Pass the result from java_library to
java_binary rule so the generated source jar ends up at the root of
buck-out folder.

Change-Id: Ie0f1e417cc8c2d5764b91d5f446528931d9462a3

java_sources.bucklet

diff --git a/java_sources.bucklet b/java_sources.bucklet
index 095a378..f20285a 100644
--- a/java_sources.bucklet
+++ b/java_sources.bucklet
@@ -28,7 +28,13 @@
     visibility = []
   ):
   java_library(
-    name = name,
+    name = name + '__tmp',
     resources = srcs,
     visibility = visibility,
   )
+  java_binary(
+    name = name,
+    deps = [
+      ':%s__tmp' % name,
+    ],
+  )