Use prek as default package if available.

[leiserfg]

I replaced the explicit calls to pre-commit with getExe so there is a single if involved. I didn't update the readme and docs references to pre-commit 'cause I'm not sure how we wanna handle it.

[sandydoo]

This looks great!

I'd like to merge this without changing the default to prek for now.
After a period of testing (perhaps in devenv), we can make the switch in a separate PR.

[sandydoo]

Suggested change

	package = lib.mkDefault (if builtins.hasAttr "prek" pkgs then pkgs.prek else pkgs.pre-commit);
	package = lib.mkDefault pre-commit;