Use maybe for WORKSPACE @bazel_worker_{api,java}

Adds an extra layer of security that we won't clobber any other repos of
the same name.

Author: mbland

scala/private/macros/workspace_compat.bzl

@@ -49,14 +49,16 @@ def workspace_compat():
         strip_prefix = "bazel-worker-api-0.0.5",
     )
 
-    _alias_repo(
+    maybe(
+        _alias_repo,
         name = "bazel_worker_api",
         aliases = {
             "worker_protocol_proto": "@bazel-worker-api//proto:worker_protocol_proto",
         },
     )
 
-    _alias_repo(
+    maybe(
+        _alias_repo,
         name = "bazel_worker_java",
         aliases = {
             "worker_protocol_java_proto": "@bazel-worker-api//java:worker_protocol_java_proto",