The use-xdg-base-directories Nix setting can be set globally in
/etc/nix/nix.conf, in which case Home Manager doesn't know about it.
Users could fix that by also setting it, redundantly, in `nix.settings`,
but then Nix issues a lot of spurious warnings about
use-xdg-base-directories being a restricted setting that untrusted users
can't pass on to the daemon.
As an alternative, users can now set `nix.assumeXdg`, which makes Home
Manager assume that use-xdg-base-directories is in effect without adding
it to the user's nix.conf file.
This is where unit files are expected to be, making GPU setup compatible
with things like selinux. Fixes#8438. The `resources/` directory was
kept because it is expected to be used in the future.
There is a need to manage XDG Base Directory system directory
environment variables in Home Manager modules. There is an existing
mechanism in `targets.genericLinux.extraXdgDataDirs', but this does not
apply to NixOS systems.
Furthermore, it is important that `XDG_CONFIG_DIRS' and `XDG_DATA_DIRS'
are set in both login shells (to support getty and SSH sessions) as well
as the systemd user manager (to propagate them to user services and
desktop environments).
The first need is addressed by adding the `xdg.systemDirs' module, which
configures lists of directory names for both `config' and `data'
directories. These are then set in
`$XDG_CONFIG_DIR/environment.d/10-home-manager.conf' and picked up by
the systemd user manager.
To make these, and other variables set in
`systemd.user.sessionVariables', available in login shells, an
additional step is added to `etc/profile.d/hm-session-vars.sh' which
exports the result of
`user-environment-generators/30-systemd-environment-d-generator' which
is shipped with systemd. The effect of this generator is to print
variables set on the systemd user manager such that shells can import
these into their environment.
