The sound distributed version control system

Move `pijul::DEFAULT_CHANNEL` to `libpijul`

This solves a circular dependency between pijul and pijul::identity, so it will be possible to split pijul::identity into its own crate. Additionally, both DOT_DIR and DEFAULT_CHANNEL are now in the same place, which should slightly improve readability.

Created by  finchie  on July 14, 2023
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