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

scripts/slave/recipes/swarming/canary.expected/linux_Debug.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/cleanup_temp.py"
     ],
     "name": "cleanup_temp"
   },
   {
@@ skipping 112 matching lines @@
     ],
     "env": {
       "GYP_DEFINES": "component=shared_library gomadir='[BUILD]/goma' target_arch=x64 test_isolation_mode=prepare use_goma=1"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
-      "[BUILD]/scripts/slave/recipe_modules/isolate/resources/find_isolated_tests.py",
+      "RECIPE_MODULE[isolate]/resources/find_isolated_tests.py",
       "--build-dir",
       "[SLAVE_BUILD]/src/out/Debug",
       "--clean-isolated-files"
     ],
     "name": "clean isolated files"
   },
   {
     "cmd": [
       "python",
       "-u",
@@ skipping 10 matching lines @@
       "[BUILD]/goma",
       "--",
       "chromium_swarm_tests"
     ],
     "name": "compile"
   },
   {
     "cmd": [
       "python",
       "-u",
-      "[BUILD]/scripts/slave/recipe_modules/isolate/resources/remove_build_metadata.py",
+      "RECIPE_MODULE[isolate]/resources/remove_build_metadata.py",
       "--build-dir",
       "[SLAVE_BUILD]/src/out/Debug",
       "--src-dir",
       "[SLAVE_BUILD]/src"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "remove_build_metadata"
   },
   {
     "cmd": [
@@ skipping 190 matching lines @@
       "@@@STEP_LOG_END@json.output@@@",
       "@@@STEP_LINK@shard #0@https://chromium-swarm-dev.appspot.com/user/task/10000@@@",
       "@@@STEP_LINK@shard #1@https://chromium-swarm-dev.appspot.com/user/task/10100@@@"
     ]
   },
   {
     "allow_subannotations": true,
     "cmd": [
       "python",
       "-u",
-      "[BUILD]/scripts/slave/recipe_modules/swarming/resources/collect_gtest_task.py",
+      "RECIPE_MODULE[swarming]/resources/collect_gtest_task.py",
       "--swarming-client-dir",
       "[SLAVE_BUILD]/src/tools/swarming_client",
       "--temp-root-dir",
       "[TMP_BASE]",
       "--merged-test-output",
       "/path/to/tmp/json",
       "--",
       "collect",
       "--swarming",
       "https://chromium-swarm-dev.appspot.com",
       "--decorate",
       "--print-status-updates",
       "--verbose",
       "--shards",
       "2",
       "dummy_target_1/Ubuntu/[dummy has/TestBuilder/571"
     ],
     "name": "dummy_target_1 on Ubuntu"
   },
   {
     "allow_subannotations": true,
     "cmd": [
       "python",
       "-u",
-      "[BUILD]/scripts/slave/recipe_modules/swarming/resources/collect_gtest_task.py",
+      "RECIPE_MODULE[swarming]/resources/collect_gtest_task.py",
       "--swarming-client-dir",
       "[SLAVE_BUILD]/src/tools/swarming_client",
       "--temp-root-dir",
       "[TMP_BASE]",
       "--merged-test-output",
       "/path/to/tmp/json",
       "--",
       "collect",
       "--swarming",
       "https://chromium-swarm-dev.appspot.com",
       "--decorate",
       "--print-status-updates",
       "--verbose",
       "--shards",
       "2",
       "dummy_target_2/Ubuntu/[dummy has/TestBuilder/571"
     ],
     "name": "dummy_target_2 on Ubuntu"
   }
 ]