Allow per deployment.nixPath .importPath

[sorki]

Useful for mixing various version of nixpkgs and OSes.

See examples/pinning.nix

[sorki]

Different take on #34, similar to NixOS/nixops#665

[johanot]

@sorki Thanks for the PR! First glance looks good to me.

To be honest, I was about to look into an adoption of the fix involving pkgs.appendOverlays as described in #34, since 19.03 is now stable.

This PR doesn't solve #34 though? i.e. the nixpkgs.pkgs option will still be ignored, right?
Question is: How much do we care?

Personally, I'd like it better if we could get nixpkgs.pkgs to work as expected, but that comes with it's own set of drawbacks. I'm interested in hearing your opinion as well @Shados . :-)

[mmahut]

Just wanted to follow up on this pull request. What is needed to move this forward?

[sorki]

The company I wrote this for has no need for this anymore as they jumped to their own solution so this can be closed - I know of one user that still uses it but they can rely on my fork / overlay with this patch. Downsides of merging this is that it increases the complexity and when actually used it increases memory consumption quite a bit due to missing memoization during evaluation.

[mmilata]

This PR is obsoleted by #67, correct?

[sorki]

Pretty much, it's not completely the same as scoped imports but looks even more flexible.