This changes the default configuration location for Home Manager
configurations from
$XDG_CONFIG_HOME/nixpkgs
to
$XDG_CONFIG_HOME/home-manager
The old location is still supported but using it will trigger a
warning message.
Fixes#3640
Internally we already managed them per-profile but exposed a global
option to maintain backwards compatibility. The benefit to having
per-profile extensions is quite large though, so it is time to switch.
Users of the global extensions option will get an error message that
indicates how to edit their configuration to work again.
