from_disk: skip intermediate dictionnary creation when building model

Before this change we would do the following :

1) translate from_disk's object into `dict`,
2) sort these dict,
3) feed the list to `Directory.from_dict`,
4) create DirectoryEntry from these dict.

Skipping the directory creating and directly creating the
DirectoryEntries provide us with a small but stable and noticeable
performance win.

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: 11 minute  56 seconds
aftere: 11 minute  50 seconds

On a profile of the same run, the `to_model` call of the from_disk's `Directory` class took the following percentage:
before: 17%
after:  15%