stack-to-nix: default.nix should be outputed on stdout

[jbgi]

default.nix typically as a different life-cycle than the others generated files, it is meant to be modified and often we don't want it in the same directory (eg. .stack.nix) as the other generated files.
Outputting default.nix on stdout would more easily allow to differentiate the two life-cycles.

On a related note, when using a directory dedicated to the generated files (eg. .stack.nix) I think it would make sense that the current pkgs.nix be default.nix instead.

[rvl]

Hmm, it's a pain to have some of the output on stdout, and the other output in some .stack.nix directory. Also if there is no template file, then we need to maintain documentation containing this boilerplate.

Since #20, I made default.nix be a template file -- previously it was what pkgs.nix is now, which is not usable by itself. It could be improved with #100.

What about having the tool compare the contents of default.nix to what it has generated. If there is a difference, don't write the file, and print a note saying "default.nix was modified, not overwriting."

[stale]

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