pyproject.toml:
Rename project to repod.

docs/*:
Change mentions of arch-repo-management to repod.

{repod,tests}/*:
Change mentions of arch-repo-management and repo_management to repod.
Change the default configuration location to `/etc/repod.conf` and the
override location to `/etc/repod.conf.d/`.

tox.ini:
Change mentions of repo_management to repod.