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

scripts/slave/recipes/chromium.expected/full_client_skia_Win_Builder.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]\\scripts\\slave\\kill_processes.py"
     ],
     "name": "taskkill"
   },
   {
@@ skipping 150 matching lines @@
       "python",
       "-u",
       "[BUILD]\\scripts\\slave\\cleanup_temp.py"
     ],
     "name": "cleanup_temp"
   },
   {
     "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 @@
       "browser_tests_run",
       "chromium_builder_tests",
       "mini_installer"
     ],
     "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\\Release",
       "--src-dir",
       "[SLAVE_BUILD]\\src"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "remove_build_metadata"
   },
   {
     "cmd": [
@@ skipping 29 matching lines @@
       "--build_revision",
       "d9c4a4d173a97ef2832b65636b4200bb93ea8ee1",
       "--factory-properties",
       "{\"blamelist\": [\"cool_dev1337@chromium.org\", \"hax@chromium.org\"], \"buildername\": \"Win Builder\", \"buildnumber\": 571, \"mastername\": \"client.skia\", \"parent_buildername\": null, \"recipe\": \"chromium\", \"slavename\": \"TestSlavename\", \"workdir\": \"/path/to/workdir/TestSlavename\"}",
       "--build-properties",
       "{\"blamelist\": [\"cool_dev1337@chromium.org\", \"hax@chromium.org\"], \"buildername\": \"Win Builder\", \"buildnumber\": 571, \"mastername\": \"client.skia\", \"parent_buildername\": null, \"recipe\": \"chromium\", \"slavename\": \"TestSlavename\", \"workdir\": \"/path/to/workdir/TestSlavename\"}"
     ],
     "name": "package build"
   }
 ]