feat: add bzl_library for defs.bzl and its dependencies (#1115)

This is so that the transitive dependencies of defs.bzl can be easily
found and validated; some Google internal tooling does this validation.

The old comment indicated bzl_library wasn't used to avoid a dependency
on skylib, however, we've since added a dependency on skylib.

Work towards #1069

python/BUILD.bazel

diff --git a/python/BUILD.bazel b/python/BUILD.bazel
index 2e275b6..e5be3e8 100644
--- a/python/BUILD.bazel
+++ b/python/BUILD.bazel
@@ -23,6 +23,7 @@
 that @rules_python//python is only concerned with the core rules.
 """
 
+load("@bazel_skylib//:bzl_library.bzl", "bzl_library")
 load(":defs.bzl", "current_py_toolchain")
 
 package(default_visibility = ["//visibility:public"])
@@ -40,8 +41,19 @@
     visibility = ["//:__pkg__"],
 )
 
+bzl_library(
+    name = "defs_bzl",
+    srcs = [
+        "defs.bzl",
+    ],
+    visibility = ["//visibility:public"],
+    deps = [
+        "//python/private:bazel_tools_bzl",
+        "//python/private:reexports_bzl",
+    ],
+)
+
 # Filegroup of bzl files that can be used by downstream rules for documentation generation
-# Using a filegroup rather than bzl_library to not give a transitive dependency on Skylib
 filegroup(
     name = "bzl",
     srcs = [

python/private/BUILD.bazel

diff --git a/python/private/BUILD.bazel b/python/private/BUILD.bazel
index 7d321eb..f327847 100644
--- a/python/private/BUILD.bazel
+++ b/python/private/BUILD.bazel
@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+load("@bazel_skylib//:bzl_library.bzl", "bzl_library")
 load("//python:versions.bzl", "print_toolchains_checksums")
 load(":stamp.bzl", "stamp_build_setting")
 
@@ -24,13 +25,30 @@
 )
 
 # Filegroup of bzl files that can be used by downstream rules for documentation generation
-# Using a filegroup rather than bzl_library to not give a transitive dependency on Skylib
 filegroup(
     name = "bzl",
     srcs = glob(["**/*.bzl"]),
     visibility = ["//python:__pkg__"],
 )
 
+bzl_library(
+    name = "reexports_bzl",
+    srcs = ["reexports.bzl"],
+    visibility = ["//python:__pkg__"],
+    deps = [":bazel_tools_bzl"],
+)
+
+# @bazel_tools can't define bzl_library itself, so we just put a wrapper around it.
+bzl_library(
+    name = "bazel_tools_bzl",
+    srcs = [
+        "@bazel_tools//tools/python:srcs_version.bzl",
+        "@bazel_tools//tools/python:toolchain.bzl",
+        "@bazel_tools//tools/python:utils.bzl",
+    ],
+    visibility = ["//python:__pkg__"],
+)
+
 # Needed to define bzl_library targets for docgen. (We don't define the
 # bzl_library target here because it'd give our users a transitive dependency
 # on Skylib.)