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%