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

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "tryserver.webrtc",
       "--builder",
       "mac",
@@ skipping 32 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",
       "[BUILD]/scripts/slave/cleanup_temp.py"
     ],
@@ skipping 325 matching lines @@
       "{\"BUILD_CONFIG\": \"Debug\", \"buildername\": \"mac\", \"issue\": 666666, \"mastername\": \"tryserver.webrtc\", \"patchset\": 1, \"recipe\": \"webrtc/standalone\", \"revision\": \"12345\", \"rietveld\": \"https://fake.rietveld.url\", \"slavename\": \"slavename\"}",
       "--annotate=gtest",
       "--test-type=voice_engine_unittests",
       "--builder-name=mac",
       "--slave-name=slavename",
       "voice_engine_unittests"
     ],
     "name": "voice_engine_unittests"
   }
 ]