Implement methods to convert the metadata available for
each version of the focused package to a standard format.
The metadata of all the versions of a focused package can
either be passed by the lister or could be obtained by an API
`fetch_metadata()` method can be overridden to get the
metadata of the focused package by the appropriate means
and convert it into a standard format.
Later, an origin is created by using the metadata provided
by the `fetch_metadata()` method.