Add providers.bzl to `shell/private:private_bzl` (#57)

This file is missing in the bzl_library target which makes starlark
generation fail for example:

```
Stardoc documentation generation failed: File /home/runner/.cache/bazel/_bazel_runner/c91220d87f26118b19246641d5266d21/sandbox/processwrapper-sandbox/251/execroot/rules_haskell/bazel-out/k8-opt-exec-2B5CBBC6/bin/docs/defs_metadata_stardoc.runfiles/rules_shell/shell/private/sh_executable.bzl imported ':providers.bzl', yet /home/runner/.cache/bazel/_bazel_runner/c91220d87f26118b19246641d5266d21/sandbox/processwrapper-sandbox/251/execroot/rules_haskell/bazel-out/k8-opt-exec-2B5CBBC6/bin/docs/defs_metadata_stardoc.runfiles/rules_shell/shell/private/providers.bzl was not found.
```