xdg-user-dirs: allow paths and define sessionVariables

[ilkecan]

Changed the type of the user directory options from str to either str path(coercedTo path toString str) to allow using path values.

The related session variable is defined for the default and the extra user directories now.

Closes #2694

Description

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• Change is backwards compatible.

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[berbiche]

AFAIK, the reason paths are not allowed is because it would lead to the entire path being imported in the nix store or prevent building on Flake configurations (Flake's can only refer to paths available in the git index).

[ilkecan]

The paths being copied to the Nix store was not a problem because currently there isn't any antiquotation happening with those values. But ideally, this should be forward compatible.

The second point was indeed a problem.

I added apply = toString to the option definitions to avoid these problems.

[sumnerevans]

let's fix this while you are at it already

Suggested change

	example = { XDG_MISC_DIR = "$HOME/Misc"; };
	example = literalExpression ''
	{
	XDG_MISC_DIR = "$HOME/Misc";
	}
	'';

[polykernel]

Suggested change

	type = with types; attrsOf (either str path);
	apply = builtins.mapAttrs (_: toString);
	type = with types; attrsOf (coercedTo path toString str);

minor nitpick, I think coercedTo is better suited here as only type coercion is happening, rather than transformation of the option value.

[ilkecan]

Thank you. I wasn't aware of the coercedTo type.

[berbiche]

Thanks again!