Buck: Make gwt_module() rule more pythonic

gwt_module() rule is a normal java_library() rule with optional
xml description of the GWT module included in resources list. The
current implementation of this rule maps hard coded all parameters
from gwt_module() to java_library().  This is not future proof, as
java_library() may become additional parameters, like provided_deps
recently and the implementation must be extended.

This change provides alternative implementation that copy parameter
list in generic way, adding GWT xml to resources list on the fly.

Inspired-By: Michael Bolin <mbolin@fb.com>
Change-Id: I7f1ee1d6b3e142f14a199e80dea4e91817b5c7d5

tools/default.defs

@@ -15,6 +15,7 @@
 # Rule definitions loaded by default into every BUCK file.
 
 include_defs('//tools/gwt-constants.defs')
+import copy
 
 def genantlr(
     name,
@@ -31,22 +32,15 @@ def genantlr(
     out = out,
   )
 
-def gwt_module(
-    name,
-    srcs,
-    gwtxml = None,
-    resources = [],
-    deps = [],
-    visibility = []):
+def gwt_module(gwtxml=None, **kwargs):
+  kw = copy.deepcopy(kwargs)
+  if 'resources' not in kw:
+    kw['resources'] = []
   if gwtxml:
-    resources = resources + [gwtxml]
-  java_library(
-    name = name,
-    srcs = srcs,
-    deps = deps,
-    resources = srcs + resources,
-    visibility = visibility,
-  )
+    kw['resources'] += [gwtxml]
+  if 'srcs' in kw:
+    kw['resources'] += kw['srcs']
+  java_library(**kw)
 
 def gerrit_extension(
     name,