Expose a Starlark library target for public pkg.bzl APIs

AI written, human reviewed.

## Problem

`rules_pkg` 1.3 still exports its public `.bzl` files, so ordinary `load()` calls continue to work. However, #1033 replaced `//pkg:bzl_srcs`, which provided `StarlarkLibraryInfo`, with the `//pkg:rules_pkg_lib` `filegroup`.

Downstream Starlark dependency tooling cannot use that filegroup as a `bzl_library` dependency. This affects tools that model the Starlark load graph, including Gazelle-generated `bzl_library` targets and lint/Stardoc analysis.

For example, a downstream file may contain:

```starlark
load("@rules_pkg//:pkg.bzl", "pkg_tar")
```

Gazelle generates a Starlark dependency for this load. With rules_pkg 1.2, `//pkg:bzl_srcs` could satisfy that dependency. With rules_pkg 1.3, `//pkg:rules_pkg_lib` is a plain `filegroup` and does not provide `StarlarkLibraryInfo`, producing an analysis error such as:

```text
does not produce any bzl_library deps files (expected .bzl or .scl)
```

## Expected behavior

A public target provides the Starlark provider for the supported `pkg.bzl` API, allowing downstream `bzl_library` targets to represent their load dependencies.

For example:

```starlark
load("@bazel_skylib//:bzl_library.bzl", "bzl_library")

bzl_library(
name = "pkg",
srcs = ["pkg.bzl"],
visibility = ["//visibility:public"],
)
```

The exact target name is not important if a documented public target is available. `//pkg:pkg` is useful because it matches Gazelle’s normal resolution of `//pkg:pkg.bzl`.

## Additional context

This is not a failure to export or load the `.bzl` files, and it does not affect normal rules_pkg execution. It is a tooling interoperability regression caused by removing the Starlark provider target in #1033.

I searched existing open and closed issues for `StarlarkLibraryInfo`, `bzl_srcs`, `starlark_library`, and Gazelle, but did not find an existing report.

Start by inspecting the definitions of //pkg:bzl_srcs and //pkg:rules_pkg_lib, then compare the provider behavior removed in #1033. Verify the documented public target can be used as a bzl_library dependency for a load of @rules_pkg//:pkg.bzl and that downstream analysis no longer reports a missing StarlarkLibraryInfo provider.

Written by the indexing model from the issue text.