Regular files were created, as the 'mode' argument of os.mknod was missing.
However, creating devices requires root; so we can't reasonably do that in
tests.

Instead, we're using /dev/null instead of creating one.
And while we're at it, let's also use /dev/zero (which, if not handled
properly, will result in an infinite read).

It should be cheap enough to do it, and it makes tests easier.

It will be useful to loaders, as they currently load the entire
content in memory before deciding to skip it.

related to T1261

Summary: Related to T1261

Test Plan: before and after: 138 tests passed (2 skipped)

Reviewers: #reviewers!

Differential Revision: https://forge.softwareheritage.org/D520

and ensure it is included in the source package.

Related to T1265.

This allows loaders to lazily load data: you can read from disk and transfer
only the contents that are really missing.

Summary:
This module is a reimplementation of swh.model.git, with the underlying goal of
replacing it and fixing T709 in the process.

Reviewers: #reviewers!

Differential Revision: https://forge.softwareheritage.org/D248