Move extra dependencies out of WORKSPACE to its own macro.

This could be useful for projects that use gRPC as an external
repository. This could not be done within the same grpc_deps.bzl due to
the problem discussed in
- https://github.com/bazelbuild/bazel/issues/1550
- https://github.com/bazelbuild/bazel/issues/1943

WORKSPACE

@@ -8,6 +8,10 @@ grpc_deps()
 
 grpc_test_only_deps()
 
+load("//bazel:grpc_extra_deps.bzl", "grpc_extra_deps")
+
+grpc_extra_deps()
+
 register_execution_platforms(
     "//third_party/toolchains:local",
     "//third_party/toolchains:local_large",
@@ -50,23 +54,3 @@ load("@io_bazel_rules_python//python:pip.bzl", "pip_repositories")
 load("@grpc_python_dependencies//:requirements.bzl", "pip_install")
 pip_repositories()
 pip_install()
-
-load("@com_google_protobuf//:protobuf_deps.bzl", "protobuf_deps")
-protobuf_deps()
-
-load("@upb//bazel:workspace_deps.bzl", "upb_deps")
-upb_deps()
-
-load("@envoy_api//bazel:repositories.bzl", "api_dependencies")
-api_dependencies()
-
-load("@io_bazel_rules_go//go:deps.bzl", "go_rules_dependencies", "go_register_toolchains")
-go_rules_dependencies()
-go_register_toolchains()
-
-
-load("@build_bazel_rules_apple//apple:repositories.bzl", "apple_rules_dependencies")
-apple_rules_dependencies()
-
-load("@build_bazel_apple_support//lib:repositories.bzl", "apple_support_dependencies")
-apple_support_dependencies()

bazel/grpc_deps.bzl

@@ -229,8 +229,16 @@ def grpc_deps():
             sha256 = "bdc8e66e70b8a75da23b79f1f8c6207356df07d041d96d2189add7ee0780cf4e",
         )
 
-    grpc_python_deps()
+    if "build_bazel_apple_support" not in native.existing_rules():
+        http_archive(
+            name = "build_bazel_apple_support",
+            urls = [
+                "https://github.com/bazelbuild/apple_support/releases/download/0.7.1/apple_support.0.7.1.tar.gz",
+            ],
+            sha256 = "122ebf7fe7d1c8e938af6aeaee0efe788a3a2449ece5a8d6a428cb18d6f88033",
+        )
 
+    grpc_python_deps()
 
 # TODO: move some dependencies from "grpc_deps" here?
 def grpc_test_only_deps():

bazel/grpc_extra_deps.bzl

@@ -0,0 +1,40 @@
+"""Loads the dependencies necessary for the external repositories defined in grpc_deps.bzl."""
+
+load("@com_google_protobuf//:protobuf_deps.bzl", "protobuf_deps")
+load("@upb//bazel:workspace_deps.bzl", "upb_deps")
+load("@envoy_api//bazel:repositories.bzl", "api_dependencies")
+load("@io_bazel_rules_go//go:deps.bzl", "go_register_toolchains", "go_rules_dependencies")
+load("@build_bazel_rules_apple//apple:repositories.bzl", "apple_rules_dependencies")
+load("@build_bazel_apple_support//lib:repositories.bzl", "apple_support_dependencies")
+
+def grpc_extra_deps():
+    """Loads the extra dependencies.
+
+    These are necessary for using the external repositories defined in
+    grpc_deps.bzl. Projects that depend on gRPC as an external repository need
+    to call both grpc_deps and grpc_extra_deps, if they have not already loaded
+    the extra dependencies. For example, they can do the following in their
+    WORKSPACE
+    ```
+    load("@com_github_grpc_grpc//bazel:grpc_deps.bzl", "grpc_deps", "grpc_test_only_deps")
+    grpc_deps()
+
+    grpc_test_only_deps()
+
+    load("@com_github_grpc_grpc//bazel:grpc_extra_deps.bzl", "grpc_extra_deps")
+
+    grpc_extra_deps()
+    ```
+    """
+    protobuf_deps()
+
+    upb_deps()
+
+    api_dependencies()
+
+    go_rules_dependencies()
+    go_register_toolchains()
+
+    apple_rules_dependencies()
+
+    apple_support_dependencies()