Tighten restrictions on directory entry names

We found a directory with an empty name in the archive: https://archive.softwareheritage.org/browse/directory/39237bec7034a819b110bfd40cfeeceacaf936d7/

We should review the restrictions that are currently placed on the directory entry names in swh-model, and add ones that we think are relevant, as these things should definitely not be accepted in the archive.

As a part of this, we should also analyze which directories currently don't match the new restrictions we settle on, and decide what to do with it (remove from the archive? yes, yes, I know...)

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Migrated from T2633 (view on Phabricator)