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

lib/gwt/BUILD

@@ -16,14 +16,14 @@
 
 java_library(
   name = 'javax-validation_src',
-  exports = ['@javax_validation_src//jar'],
+  exports = ['@javax_validation//src'],
   visibility = ['//visibility:public'],
   data = ['//lib:LICENSE-Apache2.0'],
 )
 
 java_library(
   name = 'jsinterop-annotations_src',
-  exports = ['@jsinterop_annotations_src//jar'],
+  exports = ['@jsinterop_annotations//src'],
   visibility = ['//visibility:public'],
   data = ['//lib:LICENSE-Apache2.0'],
 )