Bug 1759030 - [gecko_taskgraph] Move 'always_target' implementation o…

…utside of generator.py, r=releng-reviewers,gbrown

The `always_target` attribute is a feature which I believe I previously
mis-implemented. It doesn't belong in `generator.py` and should instead be
implemented somewhere in `target_tasks.py`. I chose to wrap the registration
decorator to guarantee backwards compatibility within Gecko. Though we may wish
to move it out to a filter in the future.

I'm making this change now to facilitate merging standalone and Gecko
taskgraphs. The `always_target` feature will be removed from standalone (as it
isn't being used and should consumers need something like it, they can roll
their own implementations).

With the feature removed from both Gecko and standalone in generator.py, this
file will now be identical across both Taskgraphs and can therefore be removed
from Gecko.

Differential Revision: https://phabricator.services.mozilla.com/D159181

taskcluster/gecko_taskgraph/generator.py

@@ -358,20 +358,7 @@ def _run(self):
             for t in full_task_graph.tasks.values()
             if t.attributes["kind"] == "docker-image"
         }
-        # include all tasks with `always_target` set
-        if parameters["tasks_for"] == "hg-push":
-            always_target_tasks = {
-                t.label
-                for t in full_task_graph.tasks.values()
-                if t.attributes.get("always_target")
-            }
-        else:
-            always_target_tasks = set()
-        logger.info(
-            "Adding %d tasks with `always_target` attribute"
-            % (len(always_target_tasks) - len(always_target_tasks & target_tasks))
-        )
-        requested_tasks = target_tasks | docker_image_tasks | always_target_tasks
+        requested_tasks = target_tasks | docker_image_tasks
         target_graph = full_task_graph.graph.transitive_closure(requested_tasks)
         target_task_graph = TaskGraph(
             {l: all_tasks[l] for l in target_graph.nodes}, target_graph