Buck: generate javadocs for plugin and extension API

  buck build api

generates now javadocs.

  buck build api_install

installs all plugin/extension related artifacts with javadocs in the
local Maven repository.

Change-Id: Ifa6a8eb469f388e16449576ff2bff01a5dce67dd

tools/maven/package.defs

diff --git a/tools/maven/package.defs b/tools/maven/package.defs
index 7306031..3171a31 100644
--- a/tools/maven/package.defs
+++ b/tools/maven/package.defs
@@ -17,11 +17,12 @@
     repository = None,
     url = None,
     jar = {},
-    src = {}):
+    src = {},
+    doc = {}):
   cmd = ['$(exe //tools/maven:mvn)', '-v', version, '-o', '$OUT']
   dep = []
 
-  for type,d in [('jar', jar), ('java-source', src)]:
+  for type,d in [('jar', jar), ('java-source', src), ('javadoc', doc)]:
     for a,t in d.iteritems():
       cmd.append('-s %s:%s:$(location %s)' % (a,type,t))
       dep.append(t)