recipe_engine: change resource API to use repo root

recipe_modules/step/example.expected/warning.json

 [
   {
     "cmd": [
       "echo",
       "Hello World"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "hello"
   },
   {
@@ skipping 16 matching lines @@
     },
     "name": "goodbye"
   },
   {
     "cmd": [
       "recipes.py",
       "--help"
     ],
     "cwd": "[SLAVE_BUILD]",
     "env": {
-      "PATH": "RECIPE_PACKAGE[recipe_engine]:%(PATH)s"
+      "PATH": "RECIPE_PACKAGE_REPO[recipe_engine]:%(PATH)s"
     },
     "name": "recipes help"
   },
   {
     "cmd": [
       "bash",
       "-c",
       "exit 3"
     ],
     "cwd": "[SLAVE_BUILD]",
@@ skipping 41 matching lines @@
       "step returned non-zero exit code: 1",
       "@@@STEP_WARNINGS@@@"
     ]
   },
   {
     "name": "$result",
     "reason": "Step('warning') failed with return_code 1",
     "status_code": 1
   }
 ]