Refactor read_pkg_file

[cdce8p]

Summary of changes

Refactor read_pkg_file from setuptools/dist.py in preparation for #2628, see #2628 (comment)
Additionally added rfc822_unescape function to reverse escaping which is applied to the Description field.
https://packaging.python.org/specifications/core-metadata/#description

/CC: @webknjaz

Pull Request Checklist

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

[webknjaz]

@cdce8p I like what you've done here. It probably deserves a misc changelog fragment too.

(FYI I don't have a merge privilege on this repo so waiting for @jaraco to accept this patch)

[cdce8p]

@webknjaz I didn't add a changelog entry, since this change shouldn't effect the end user. If you would like that I do it anyway, please let me know.

[webknjaz]

@cdce8p yep, that makes sense to me. I'll leave it to @jaraco to decide, then.

[jaraco]

Looks good to me.