docs: //pkg:bzl_srcs update deps.

This change updates deps for //pkg:bzl_srcs to include paths and
rules_python, which comes from the individual rules.

This bzl_library is next to the .bzl files. Users of rules_pkg may build
stardoc for their own extensions by relying on this bzl_library
directly.

This change deletes the unnecessary starlark_library rule, because
starlark_library itself already creates the non-dev dependency from
rules_pkg to skylib. A user of rules_pkg will have to have skylib
anyways. So let's just use skylib's bzl_library directly. It is okay
to put non .bzl files in bzl_library.

Test: locally update latest.md, no significant difference other than
  small changes due to the file being outdated in the first place.
Link: #897

doc_build/BUILD

@@ -116,8 +116,9 @@ bzl_library(
     name = "rules_pkg_lib",
     srcs = [
         "//:version.bzl",
+    ],
+    deps = [
         "//pkg:bzl_srcs",
-        "@bazel_skylib//lib:paths",
     ],
     visibility = ["//visibility:public"],
 )

pkg/BUILD

@@ -12,9 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-load("@rules_pkg//pkg/private:make_starlark_library.bzl", "starlark_library")
-
 # -*- coding: utf-8 -*-
+load("@bazel_skylib//:bzl_library.bzl", "bzl_library")
 load("@rules_python//python:defs.bzl", "py_binary", "py_library")
 
 package(default_applicable_licenses = ["//:license"])
@@ -51,7 +50,7 @@ filegroup(
     ],
 )
 
-starlark_library(
+bzl_library(
     name = "bzl_srcs",
     srcs = [
         ":standard_package",
@@ -62,6 +61,10 @@ starlark_library(
         "//pkg/private/zip:standard_package",
         "//pkg/releasing:standard_package",
     ],
+    deps = [
+        "@rules_python//python:defs_bzl",
+        "@bazel_skylib//lib:paths",
+    ],
     visibility = ["//visibility:public"],
 )