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.

* _loader/types.py: While initializing `Dependency` objects, attempt to
                    split filenames, only if no `junction` was specified
                    explicitly. If a `junction` was specified, then
                    filenames with `:` in their names will result in an
                    error.

* tests/frontend/buildcheckout.py: Add tests to ensure the above.

* _versions.py: Bump BST_FORMAT_VERSION.

Fixes #809.