chore: Add invert_repositories function

Generic function for working around
bazelbuild/bazel#5356

nixpkgs/nixpkgs.bzl

@@ -167,21 +167,31 @@ _nixpkgs_package = repository_rule(
 )
 
 def nixpkgs_package(*args, **kwargs):
+    invert_repositories(_nixpkgs_package, *args, **kwargs)
+
+def invert_dict(dict):
+    """Swap the keys and values of a dict − assuming that all values are
+    distinct
+    """
+    return {value: key for (key, value) in dict.items()}
+
+def invert_repositories(f, *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).
-    if "repositories" in kwargs:
-        inversed_repositories = {value: key for (key, value) in kwargs["repositories"].items()}
-        kwargs.pop("repositories")
-        _nixpkgs_package(
-            repositories = inversed_repositories,
-            *args,
-            **kwargs
-        )
-    else:
-        _nixpkgs_package(*args, **kwargs)
+    hasRepository = "repository" in kwargs and kwargs["repository"] != None
+    hasRepositories = "repositories" in kwargs and kwargs["repositories"] != None
+    if hasRepository and hasRepositories:
+        fail("Specify one of 'repository' or 'repositories' (but not both).")
+    if hasRepositories:
+        inversed_repositories = invert_dict(kwargs["repositories"])
+        kwargs["repositories"] = inversed_repositories
+    if hasRepository:
+        repository = kwargs.pop("repository")
+        kwargs["repositories"] = { repository: "nixpkgs" }
+    f(*args, **kwargs)
 
 def nixpkgs_cc_autoconf_impl(repository_ctx):
     cpu_value = get_cpu_value(repository_ctx)