Add simple depends_on API.

recipe_engine/package.py

 import ast
 import collections
 import contextlib
 import copy
 import functools
 import itertools
 import logging
 import os
 import subprocess
 import sys
@@ skipping 599 matching lines @@
     for repo in self._repos.values():
       for subdir in repo.recipe_dirs:
         yield str(subdir)
 
   @property
   def all_module_dirs(self):
     for repo in self._repos.values():
       for subdir in repo.module_dirs:
         yield str(subdir)
 
+  @property
+  def engine_recipes_py(self):
+    return os.path.join(self._context.repo_root, 'recipes.py')
+
 
 def _run_cmd(cmd, cwd=None):
   cwd_str = ' (in %s)' % cwd if cwd else ''
   logging.info('%s%s', cmd, cwd_str)
   subprocess.check_call(cmd, cwd=cwd)
 
 
 def _merge2(xs, ys, compare=lambda x, y: x <= y):
   """Merges two sorted iterables, preserving sort order.
 
@@ skipping 62 matching lines @@
   >>> d = { 'x': 1, 'y': 2 }
   >>> sorted(_updated(d, { 'y': 3, 'z': 4 }).items())
   [('x', 1), ('y', 3), ('z', 4)]
   >>> sorted(d.items())
   [('x', 1), ('y', 2)]
   """
 
   d = copy.copy(d)
   d.update(updates)
   return d