'Get the filename for the module we should load for a particular
package.  Packages for a Repo live in
``$root/<package_name>/package.py``
This will return a proper package.py path even if the
package doesn\'t exist yet, so callers will need to ensure
the package exists before importing.'

@_autospec
def filename_for_package_name(self, spec):

    self._check_namespace(spec)
    pkg_dir = self.dirname_for_package_name(spec.name)
    return join_path(pkg_dir, package_file_name)
