Make workspace_deps.bzl overridable using maybe()

Make it possible for a user to define a different version of the upb
dependencies by wrapping them in maybe(). Then the upb dependencies will
not be added if they previously have been declared.

bazel/workspace_deps.bzl

@@ -1,33 +1,39 @@
 
 load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
 load("@bazel_tools//tools/build_defs/repo:git.bzl", "git_repository")
+load("@bazel_tools//tools/build_defs/repo:utils.bzl", "maybe")
 load("//bazel:repository_defs.bzl", "bazel_version_repository")
 
 def upb_deps():
-    bazel_version_repository(
+    maybe(
+        bazel_version_repository,
         name = "upb_bazel_version",
     )
 
-    git_repository(
+    maybe(
+        git_repository,
         name = "absl",
         commit = "070f6e47b33a2909d039e620c873204f78809492",
         remote = "https://github.com/abseil/abseil-cpp.git",
         shallow_since = "1541627663 -0500",
     )
 
-    git_repository(
+    maybe(
+        git_repository,
         name = "com_google_protobuf",
         remote = "https://github.com/protocolbuffers/protobuf.git",
         commit = "d41002663fd04325ead28439dfd5ce2822b0d6fb",
     )
 
-    http_archive(
+    maybe(
+        http_archive,
         name = "bazel_skylib",
         strip_prefix = "bazel-skylib-master",
         urls = ["https://github.com/bazelbuild/bazel-skylib/archive/master.tar.gz"],
     )
 
-    http_archive(
+    maybe(
+        http_archive,
         name = "zlib",
         build_file = "@com_google_protobuf//:third_party/zlib.BUILD",
         sha256 = "629380c90a77b964d896ed37163f5c3a34f6e6d897311f1df2a7016355c45eff",