Use virtual artifact directory to stage and extract metadata

This branch eliminates extract directories (#913 (closed), #920 (closed), #921 (closed)). Artifact access for staging and extracting metadata is handled solely via CasBasedDirectory. Initial tests have shown a reduction in staging time by almost 40%.

tests/integration/pullbuildtrees.py

@@ -22,7 +22,6 @@ DATA_DIR = os.path.join(
 # cleared as just forcefully removing the refpath leaves dangling objects.
 def default_state(cli, tmpdir, share):
     shutil.rmtree(os.path.join(str(tmpdir), 'cas'))
-    shutil.rmtree(os.path.join(str(tmpdir), 'extract'))
     cli.configure({
         'artifacts': {'url': share.repo, 'push': False},
         'cachedir': str(tmpdir),
@@ -70,17 +69,11 @@ def test_pullbuildtrees(cli2, tmpdir, datafiles):
 
         # Pull artifact with default config, then assert that pulling
         # with buildtrees cli flag set creates a pull job.
-        # Also assert that the buildtree is added to the artifact's
-        # extract dir
         result = cli2.run(project=project, args=['artifact', 'pull', element_name])
         assert element_name in result.get_pulled_elements()
-        elementdigest = share1.has_artifact('test', element_name, cli2.get_element_key(project, element_name))
-        buildtreedir = os.path.join(str(tmpdir), 'extract', 'test', 'autotools-amhello',
-                                    elementdigest.hash, 'buildtree')
-        assert not os.path.isdir(buildtreedir)
+        assert share1.has_artifact('test', element_name, cli2.get_element_key(project, element_name))
         result = cli2.run(project=project, args=['--pull-buildtrees', 'artifact', 'pull', element_name])
         assert element_name in result.get_pulled_elements()
-        assert os.path.isdir(buildtreedir)
         default_state(cli2, tmpdir, share1)
 
         # Pull artifact with pullbuildtrees set in user config, then assert
@@ -138,7 +131,6 @@ def test_pullbuildtrees(cli2, tmpdir, datafiles):
         result = cli2.run(project=project, args=['--pull-buildtrees', 'artifact', 'push', element_name])
         assert "Attempting to fetch missing artifact buildtrees" in result.stderr
         assert element_name not in result.get_pulled_elements()
-        assert not os.path.isdir(buildtreedir)
         assert element_name not in result.get_pushed_elements()
         assert not share3.has_artifact('test', element_name, cli2.get_element_key(project, element_name))
 
@@ -150,7 +142,6 @@ def test_pullbuildtrees(cli2, tmpdir, datafiles):
         result = cli2.run(project=project, args=['--pull-buildtrees', 'artifact', 'push', element_name])
         assert "Attempting to fetch missing artifact buildtrees" in result.stderr
         assert element_name in result.get_pulled_elements()
-        assert os.path.isdir(buildtreedir)
         assert element_name in result.get_pushed_elements()
         assert share3.has_artifact('test', element_name, cli2.get_element_key(project, element_name))