Bug 1341214 - Define GECKO in a single location. r=dustin

Instead of every file trying to get the top source directory having an
ad-hoc definition that gets wrong if the files gets moved around for
some reason, define it in a more central location.

taskcluster/taskgraph/decision.py

@@ -18,6 +18,7 @@ from .create import create_tasks
 from .parameters import Parameters
 from .taskgraph import TaskGraph
 from actions import render_actions_json
+from . import GECKO
 
 from taskgraph.util.templates import Templates
 from taskgraph.util.time import (
@@ -28,7 +29,6 @@ from taskgraph.util.time import (
 logger = logging.getLogger(__name__)
 
 ARTIFACTS_DIR = 'artifacts'
-GECKO = os.path.realpath(os.path.join(__file__, '..', '..', '..'))
 
 # For each project, this gives a set of parameters specific to the project.
 # See `taskcluster/docs/parameters.rst` for information on parameters.