Bazel: use a custom maven_jar rule

Using existing download_file.py from custom maven_jar Skylark rule
gives us the following advantages:

* fetching of sources (needed for GWT JSNI build and IDE integration)
* server support for proxies
* global, per-user cache of artifacts
* support for local maven repository (~/.m2)

Change-Id: Ic246dd9298775a6819f05453c601c688d8cb7ab9

gerrit-patch-jgit/BUILD

@@ -27,12 +27,12 @@ gwt_module(
 genrule2(
   name = 'jgit_edit_src',
   cmd = ' && '.join([
-    'unzip -qd $$TMP $(location @jgit_src//file) ' +
+    'unzip -qd $$TMP $(location @jgit//src) ' +
       'org/eclipse/jgit/diff/Edit.java',
     'cd $$TMP',
     'zip -Dq $$ROOT/$@ org/eclipse/jgit/diff/Edit.java',
   ]),
-  tools = ['@jgit_src//file'],
+  tools = ['@jgit//src'],
   outs = [ 'edit.srcjar' ],
 )