Return contents of file corresponding to *pack_uri* in package
        directory.

def blob_for(self, pack_uri):
        """
        Return contents of file corresponding to *pack_uri* in package
        directory.
        """
        path = os.path.join(self._path, pack_uri.membername)
        with open(path, 'rb') as f:
            blob = f.read()
        return blob