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

scripts/slave/recipes/chromium_trybot_legacy.expected/swarming_missing_isolated.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "tryserver.chromium.linux",
       "--builder",
       "linux_chromium_asan_rel",
@@ skipping 197 matching lines @@
     ],
     "env": {
       "GYP_DEFINES": "asan=1 clang=1 component=static_library dcheck_always_on=1 fastbuild=1 gomadir='[BUILD]/goma' lsan=1 target_arch=x64 test_isolation_mode=prepare use_allocator=none use_goma=1"
     },
     "name": "gclient runhooks (with patch)"
   },
   {
     "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/Release",
       "--clean-isolated-files"
     ],
     "name": "clean isolated files"
   },
   {
     "cmd": [
       "python",
       "-u",
@@ skipping 13 matching lines @@
       "base_unittests_run",
       "browser_tests",
       "browser_tests_run"
     ],
     "name": "compile (with patch)"
   },
   {
     "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/Release",
       "--src-dir",
       "[SLAVE_BUILD]/src"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "remove_build_metadata"
   },
   {
     "cmd": [
@@ skipping 155 matching lines @@
       "@@@STEP_LOG_LINE@json.output@  \"valid\": true@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@"
     ]
   },
   {
     "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.appspot.com",
@@ skipping 35 matching lines @@
       "@@@STEP_FAILURE@@@",
       "@@@SET_BUILD_PROPERTY@failure_hash@\"639c9bb258abcb602fb0b12e3f6aa8e0d794a562\"@@@"
     ]
   },
   {
     "name": "$final_result",
     "reason": "1 out of 2 aggregated steps failed. Failures: Step('browser_tests') failed with return_code 1",
     "status_code": 1
   }
 ]