use different prefix than `src = ./.`

[MMesch]

it would be nice to be able to be able to put the stack2nix.nix file into a /nix directory and use a prefix such as src = ../. for the packages.

[domenkozar]

Duplicates #119

[MMesch]

I wonder if it really is a duplicate. The other issue seems to focus on filterSource and not on changing the actual prefix.

[domenkozar]

In both cases there's a need for overriding the default src value.

[MMesch]

true, they are closely related. In any case I think it would be a nice feature to have. It would be a shame to forget considering it when the other issue is addressed.