Dissolve top level gitiles-* directories

The current layout of having gitiles-* directories with separate java
source trees dates back to the bad era of Maven, when this specific
layout was necessary to decompose the build into separate pom.xml files.

Moreover, src/{main,test}/java are also Maven artifacts, so that we go
even further and create three top level directories:

* java
* javatests
* resources

Change-Id: I4421096428db1e3de019a9b6c1253217cf7e4fbe

tools/bazlets.bzl

diff --git a/tools/bazlets.bzl b/tools/bazlets.bzl
index e14e488..b5c240c 100644
--- a/tools/bazlets.bzl
+++ b/tools/bazlets.bzl
@@ -1,7 +1,7 @@
 NAME = "com_googlesource_gerrit_bazlets"
 
 def load_bazlets(
-    commit,
+    commit = None,
     local_path = None
   ):
   if not local_path: