Support config paths under /usr/

Describe the feature you'd like see implemented in sudo-rs

sudoers configs should be able to be read from /usr/lib/sudo/sudoers and /usr/lib/sudo/sudoers.d/

What problem can be solved with this feature?

In postmarketOS, we use sudo-rs as our default sudo implementation. This has worked fine so far, but we are attempting to limit configs installed to /etc/, reserving it as a sysadmin configuration directory per the concept of hermetic-usr. This is in conflict with sudo-rs, which only supports loading configs from /etc/.

Describe alternatives you've considered

Because sudo-rs only supports loading sudoers from /etc/, the only way we can handle sudoers is to use tmpfiles.d to copy the file from /usr/share/factory/etc/ on first-boot. It would be much easier if sudo-rs was able to read sudoers from a path under /usr/, removing the need for complex config file logic.

Additional context

• https://github.com/uapi-group/specifications/issues/76 (specifically the section at the top on sudo)
• https://uapi-group.org/specifications/specs/configuration_files_specification/ (bit of an explanation on hermetic-usr)