loader: Allow dependencies to use ":" to refer to junctioned elements

This will allow cross-junction dependencies to be listed as strings on a
single line.

Fixes #809.

buildstream/_loader/loadelement.py

@@ -150,7 +150,13 @@ def _extract_depends_from_node(node, *, key=None):
         dep_provenance = _yaml.node_get_provenance(node, key=key, indices=[index])
 
         if isinstance(dep, str):
-            dependency = Dependency(dep, provenance=dep_provenance, dep_type=default_dep_type)
+            junction_path = dep.rsplit(":", 1)
+            filename = junction_path[-1]
+            junction = None if len(junction_path) == 1 else junction_path[-2]
+            dependency = Dependency(filename,
+                                    junction=junction,
+                                    dep_type=default_dep_type,
+                                    provenance=dep_provenance)
 
         elif isinstance(dep, Mapping):
             if default_dep_type:

tests/frontend/buildcheckout.py

@@ -593,3 +593,45 @@ def test_build_checkout_cross_junction(datafiles, cli, tmpdir):
 
     filename = os.path.join(checkout, 'etc', 'animal.conf')
     assert os.path.exists(filename)
+
+
+@pytest.mark.datafiles(DATA_DIR)
+def test_build_junction_short_notation(cli, tmpdir, datafiles):
+    project = os.path.join(datafiles.dirname, datafiles.basename)
+    subproject_path = os.path.join(project, 'files', 'sub-project')
+    junction_path = os.path.join(project, 'elements', 'junction.bst')
+    element_path = os.path.join(project, 'elements', 'junction-dep.bst')
+    workspace = os.path.join(cli.directory, 'workspace')
+    checkout = os.path.join(cli.directory, 'checkout')
+
+    # Create a repo to hold the subproject and generate a junction element for it
+    ref = generate_junction(tmpdir, subproject_path, junction_path)
+
+    # Create a stack element to depend on a cross junction element, using
+    # colon (:) as the separator
+    element = {
+        'kind': 'stack',
+        'depends': ['junction.bst:import-etc.bst']
+    }
+    _yaml.dump(element, element_path)
+
+    # Now try to build it, this should automatically result in fetching
+    # the junction itself at load time.
+    result = cli.run(project=project, args=['build', 'junction-dep.bst'])
+    result.assert_success()
+
+    # Assert that it's cached now
+    assert cli.get_element_state(project, 'junction-dep.bst') == 'cached'
+
+    # Now check it out
+    result = cli.run(project=project, args=[
+        'checkout', 'junction-dep.bst', checkout
+    ])
+    result.assert_success()
+
+    # Assert the content of /etc/animal.conf
+    filename = os.path.join(checkout, 'etc', 'animal.conf')
+    assert os.path.exists(filename)
+    with open(filename, 'r') as f:
+        contents = f.read()
+    assert contents == 'animal=Pony\n'