from_disk: only build a model object once
Before this change, a Directory object was built to compute the `id` of
we fed to the Directory object we built for `to_model`.
We tested this change on simple information of the Mercurial loader,
with a noop-loader stockage:
swh loader run mercurial https://foss.heptapod.net/mercurial/mercurial-devel directory=/data/repos/mercurial-devel
= Median time of 3 run =
before: 17 minutes 48 seconds
after: 12 minutes 59 seconds
On a profile of the same run, the `to_model` call of the from_disk's `Directory` class took the following percentage:
before: 43%
after: 24%