respect XDG by default on posixlikes

[asottile]

👋 I'm down a bit of a rabbit hole -- let me explain first where I'm coming from and then my quick proposal

I am building a docker image which intentionally does not provide a writable home directory -- but instead provides a bunch of writable $XDG_... directories. I've noticed that cpan uses this module to determine where to write its configuration on first startup but File::HomeDir->my_data is always returning the home directory

I can work around this by installing xdg-user-dirs manually, or providing a placeholder executable by the same name -- but I don't think that executable should be necessary at all since most modern posixlikes support the XDG "standard"

personally, I would like to see the FreeDeskptop implementation be the default on posixlikes but that might be too aggressive of a change -- perhaps a more middle ground would be to have the default Unix implementation respect the $XDG_* variables even if the xdg-user-dirs executable isn't present (since it's kind of rare to install this in a docker image, even if you want the functionality)

the directories from xdg-user-dirs are also easily derivable without running the xdg-user-dir subprocess -- since they're specified: https://wiki.archlinux.org/title/XDG_user_directories