Remove dirname hack from nix_path concatenation

Previously we applied `dirname` to each path, which meant we could
only use nix files named `default.nix` (because of nix’s defaulting to
that file). Now we can name files directly (see the `WORKSPACE` file
for an example that calls `nixpksgs-pin.nix`).

WORKSPACE

@@ -19,38 +19,38 @@ nixpkgs_package(
 
 nixpkgs_package(
   name = "hello",
-  repositories = { "nixpkgs": "//:nix/default.nix" }
+  repositories = { "nixpkgs": "//:nixpkgs.nix" }
 )
 
 nixpkgs_package(
   name = "expr-test",
   nix_file_content = "let pkgs = import <nixpkgs> {}; in pkgs.hello",
-  repositories = { "nixpkgs": "//:nix/default.nix" }
+  repositories = { "nixpkgs": "//:nixpkgs.nix" }
 )
 
 nixpkgs_package(
   name = "attribute-test",
   attribute_path = "hello",
-  repositories = { "nixpkgs": "//:nix/default.nix" }
+  repositories = { "nixpkgs": "//:nixpkgs.nix" }
 )
 
 nixpkgs_package(
   name = "expr-attribute-test",
   nix_file_content = "import <nixpkgs> {}",
   attribute_path = "hello",
-  repositories = { "nixpkgs": "//:nix/default.nix" },
+  repositories = { "nixpkgs": "//:nixpkgs.nix" },
 )
 
 nixpkgs_package(
   name = "nix-file-test",
   nix_file = "//tests:nixpkgs.nix",
   attribute_path = "hello",
-  repositories = { "nixpkgs": "//:nix/default.nix" },
+  repositories = { "nixpkgs": "//:nixpkgs.nix" },
 )
 
 nixpkgs_package(
   name = "nix-file-deps-test",
   nix_file = "//tests:hello.nix",
   nix_file_deps = ["//tests:pkgname.nix"],
-  repositories = { "nixpkgs": "//:nix/default.nix" },
+  repositories = { "nixpkgs": "//:nixpkgs.nix" },
 )

nixpkgs/nixpkgs.bzl

@@ -79,12 +79,9 @@ def _nixpkgs_package_impl(ctx):
   # unless a pinned nixpkgs is set in the `nix_file` attribute.
   nix_path = ""
   if repositories:
-    # XXX Another hack: the repository label typically resolves to
-    # some top-level package in the external workspace. So we use
-    # dirname to get the actual workspace path.
     nix_path = ":".join(
-      [(path_name + "=" + str(ctx.path(path).dirname))
-         for (path, path_name) in repositories.items()])
+      [(path_name + "=" + str(ctx.path(target)))
+         for (target, path_name) in repositories.items()])
   elif not (ctx.attr.nix_file or ctx.attr.nix_file_content):
     fail(strFailureImplicitNixpkgs)
 

shell.nix

@@ -1,4 +1,4 @@
-{ pkgs ? import ./nix/default.nix {} }:
+{ pkgs ? import ./nixpkgs.nix {} }:
 
 with pkgs;