Give a nicer interface to nixpkgs_package

Make the `repositories` argument of the form `{ path_element_name: label
}` instead of `{ label: path_element_name }`

WORKSPACE

@@ -4,37 +4,37 @@ load("//nixpkgs:nixpkgs.bzl", "nixpkgs_git_repository", "nixpkgs_package")
 
 # For tests
 
-nixpkgs_package(name = "hello", repositories = { "//:nix/default.nix": "nixpkgs" })
+nixpkgs_package(name = "hello", repositories = { "nixpkgs": "//:nix/default.nix" })
 
 nixpkgs_package(
   name = "expr-test",
   nix_file_content = "let pkgs = import <nixpkgs> {}; in pkgs.hello",
-  repositories = { "//:nix/default.nix": "nixpkgs" }
+  repositories = { "nixpkgs": "//:nix/default.nix" }
 )
 
 nixpkgs_package(
   name = "attribute-test",
   attribute_path = "hello",
-  repositories = { "//:nix/default.nix": "nixpkgs" }
+  repositories = { "nixpkgs": "//:nix/default.nix" }
 )
 
 nixpkgs_package(
   name = "expr-attribute-test",
   nix_file_content = "import <nixpkgs> {}",
   attribute_path = "hello",
-  repositories = { "//:nix/default.nix": "nixpkgs" },
+  repositories = { "nixpkgs": "//:nix/default.nix" },
 )
 
 nixpkgs_package(
   name = "nix-file-test",
   nix_file = "//tests:nixpkgs.nix",
   attribute_path = "hello",
-  repositories = { "//:nix/default.nix": "nixpkgs" },
+  repositories = { "nixpkgs": "//:nix/default.nix" },
 )
 
 nixpkgs_package(
   name = "nix-file-deps-test",
   nix_file = "//tests:hello.nix",
   nix_file_deps = ["//tests:pkgname.nix"],
-  repositories = { "//:nix/default.nix": "nixpkgs" },
+  repositories = { "nixpkgs": "//:nix/default.nix" },
 )

nixpkgs/nixpkgs.bzl

@@ -116,7 +116,7 @@ def _nixpkgs_package_impl(ctx):
   _symlink_children(output_path, ctx)
 
 
-nixpkgs_package = repository_rule(
+_nixpkgs_package = repository_rule(
   implementation = _nixpkgs_package_impl,
   attrs = {
     "attribute_path": attr.string(),
@@ -132,6 +132,19 @@ nixpkgs_package = repository_rule(
   local = True,
 )
 
+def nixpkgs_package(repositories, *args, **kwargs):
+    # Because of https://github.com/bazelbuild/bazel/issues/5356 we can't
+    # directly pass a dict from strings to labels to the rule (which we'd like
+    # for the `repositories` arguments), but we can pass a dict from labels to
+    # strings. So we swap the keys and the values (assuming they all are
+    # distinct).
+    inversed_repositories = { value: key for (key, value) in repositories.items() }
+    _nixpkgs_package(
+        repositories = inversed_repositories,
+        *args,
+        **kwargs
+    )
+
 def _symlink_children(target_dir, rep_ctx):
   """Create a symlink to all children of `target_dir` in the current
   build directory."""