Changed recipe_modules base paths to be abstract entities in their own right.

scripts/slave/recipes/webrtc/standalone.expected/client_webrtc_android_gn.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "client.webrtc",
       "--builder",
       "Android GN",
@@ skipping 26 matching lines @@
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@",
       "@@@SET_BUILD_PROPERTY@got_revision@\"f27fede2220bcd326aee3e86ddfd4ebd0fe58cb9\"@@@",
       "@@@SET_BUILD_PROPERTY@got_revision_cp@\"refs/heads/master@{#170242}\"@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
-      "[BUILD]/scripts/slave/recipe_modules/webrtc/resources/cleanup_files.py",
+      "RECIPE_MODULE[webrtc]/resources/cleanup_files.py",
       "[SLAVE_BUILD]/src/out"
     ],
     "name": "clean test output files"
   },
   {
     "cmd": [
       "python",
       "-u",
       "\nimport shutil, sys, os\nshutil.rmtree(sys.argv[1], True)\nshutil.rmtree(sys.argv[2], True)\ntry:\n  os.remove(sys.argv[3])\nexcept OSError:\n  pass\nfor base, _dirs, files in os.walk(sys.argv[4]):\n  for f in files:\n    if f.endswith('.pyc'):\n      os.remove(os.path.join(base, f))\n",
       "[SLAVE_BUILD]/src/out/Release/debug_info_dumps",
@@ skipping 60 matching lines @@
       "[BUILD]/goma",
       "--",
       "all"
     ],
     "env": {
       "PATH": "[SLAVE_BUILD]/src/third_party/android_tools/sdk/platform-tools:[SLAVE_BUILD]/src/build/android:%(PATH)s"
     },
     "name": "compile"
   }
 ]