This reduce the number of function call and should be faster.

The mashup of blind optimisation in the previous changeset yield some
interesting results in total.

It would be insightful to measure them individually, but that would
take more time than we currently have.

When testing all the validator changes on our previous "benchmark" we
see quite interesting improvement.

    swh loader run mercurial https://foss.heptapod.net/mercurial/mercurial-devel directory=/data/repos/mercurial-devel

= Median time of 3 run =
base:   17 minutes 48 seconds
before: 11 minutes 50 seconds
after:  11 minutes 11 seconds

On a profile of the same run, the `to_model` call of the from_disk's `Directory` class took the following percentage:
base:   43%
before: 15%
after:  11%