Make documentation bzl_library publicly visible

This allows anyone who depends on `rules_jvm_external` and imports
"something" from these rules into their own rules to be able to
generate stardoc of their own functions.

I've filed bazelbuild/stardoc#93 to track
this issue. Once it's fixed, we can close this.

BUILD

@@ -33,6 +33,9 @@ stardoc(
 bzl_library(
  name = "implementation",
  srcs = [
+ ":coursier.bzl",
+ ":defs.bzl",
+ ":specs.bzl",
  "//:private/coursier_utilities.bzl",
  "//:private/dependency_tree_parser.bzl",
  "//:private/proxy.bzl",
@@ -48,9 +51,9 @@ bzl_library(
  "//private/rules:pom_file.bzl",
  "//settings:stamp_manifest.bzl",
  "//third_party/bazel_json/lib:json_parser.bzl",
-  ":coursier.bzl",
-  ":defs.bzl",
- ":specs.bzl",
+ ],
+ visibility = [
+ "//visibility:public",
  ],
 )