Remove reliance on ensure_directory and parse_requirements from pkg_resources

[jaraco]

• Add importlib_resources and importlib_metadata to vendored packages.
• Add module for selectively loading importlib modules.
• Move ensure_directory into setuptools.
• Refresh importlib_metadata to 4.10.1.
• Vendor jaraco.text with setuptools.
• In build_meta, remove dependency on pkg_resources.
• Pin vendored importlib dependencies for consistency.
• Update changelog.

Summary of changes

Progress toward #3085.

Pull Request Checklist

• Changes have tests
• News fragment added in changelog.d/.
  (See documentation for details)

[jaraco]

I was planning to create one giant pull request to migrate everything off pkg_resources, but I realized that in the interest of stability and incrementalism, it's probably better to merge pieces of functionality in pieces. This piece implements a few of the early commits toward the effort.