_artifactelement.py: New module containing ArtifactElement class

The ArtifactElement class is to be used when we want to pass a
dummy artifact to our scheduler, typically this will be when we
are directly working with artifact refs.

buildstream/_artifactelement.py

+#
+#  Copyright (C) 2019 Bloomberg Finance LP
+#
+#  This program is free software; you can redistribute it and/or
+#  modify it under the terms of the GNU Lesser General Public
+#  License as published by the Free Software Foundation; either
+#  version 2 of the License, or (at your option) any later version.
+#
+#  This library is distributed in the hope that it will be useful,
+#  but WITHOUT ANY WARRANTY; without even the implied warranty of
+#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.	 See the GNU
+#  Lesser General Public License for more details.
+#
+#  You should have received a copy of the GNU Lesser General Public
+#  License along with this library. If not, see <http://www.gnu.org/licenses/>.
+#
+#  Authors:
+#        James Ennis <james.ennis@codethink.co.uk>
+from ._exceptions import ArtifactElementError
+
+
+# ArtifactElement()
+#
+# Object to be used for directly processing an artifact
+#
+# Args:
+#    context (Context): The Context object
+#    ref (str): The artifact ref
+#
+class ArtifactElement():
+    def __init__(self, context, ref):
+        try:
+            project_name, element, key = ref.split('/', 2)
+        except ValueError:
+            raise ArtifactElementError("Artifact: {} is not of the expected format".format(ref))
+
+        self._project_name = project_name
+        self._element = element
+        self._key = key
+        self._context = context
+
+    def get_artifact_name(self):
+        return '{0}/{1}/{2}'.format(self._project_name, self._element, self._key)
+
+    def _cached(self):
+        return self._context.artifactcache.contains_ref(self.get_artifact_name())