Plugins are already allowed to fail to load, let's be even more tolerant of them failing to load by ignoring inaccessible ones with a warning. This change also significantly reworks the docs for plugins. This is a tiny extremely partial backport of the C API at https://github.com/NixOS/nix/pull/8699. Fixes: https://git.lix.systems/lix-project/lix/issues/740 CC: https://git.lix.systems/lix-project/lix/issues/359 Change-Id: If4ee20c3daaf26c8184a415eef3e20ca5b5e7aef