Add a state to the lister to store the ``last_seen_commit`` as a Git
commit hash.
Use Dulwich to retrieve a Git commit walker since
``last_seen_commit`` if any.
For each commit detect if it is a new package or a new package
version commit and returns its origin with commit date as
last_update.
This module introduce Julia Lister.
It retrieves Julia packages origins from the Julia General Registry, a Git
repository made of per package directory with Toml definition files.
