Make the Nix search path declarative

Nix search path lookups like <nixpkgs> are now desugared to ‘findFile
nixPath <nixpkgs>’, where ‘findFile’ is a new primop. Thus you can
override the search path simply by saying

  let
    nixPath = [ { prefix = "nixpkgs"; path = "/my-nixpkgs"; } ];
  in ... <nixpkgs> ...

In conjunction with ‘scopedImport’ (commit
c273c15cb13bb86420dda1e5341a4e19517532b5), the Nix search path can be
propagated across imports, e.g.

  let

    overrides = {
      nixPath = [ ... ] ++ builtins.nixPath;
      import = fn: scopedImport overrides fn;
      scopedImport = attrs: fn: scopedImport (overrides // attrs) fn;
      builtins = builtins // overrides;
    };

  in scopedImport overrides ./nixos

1 files changed, 1 insertions, 1 deletions

diff --git a/tests/lang/eval-okay-search-path.exp b/tests/lang/eval-okay-search-path.exp
index d0bc8c5e86b2..ad05904ba1f7 100644
--- a/tests/lang/eval-okay-search-path.exp
+++ b/tests/lang/eval-okay-search-path.exp
@@ -1 +1 @@
-"abcc"
+"abcca"