Implement PEP 753 normalization for Home-Page fallback

[ichard26]

We treat "homepage" as a well-known project URL, using it to populate the Home-Page core metadata field if it's undefined. This patch updates the normalization logic to follow PEP 753.

Feedback wanted: The logic emitting the raw project URLs was left unchanged as we don't really care for well-known vs. boring URLs here. I believe this is the right call, but I can be convinced to also apply normalization here (although that could break anyone who is using pip show to inspect metadata which is actually a valid use-case as the show output format is documented.)

$ pip show pip --verbose
Name: pip
Version: 25.1.dev0
Summary: The PyPA recommended tool for installing Python packages.
Home-page: https://pip.pypa.io/
Author: 
Author-email: The pip developers <[email protected]>
License: MIT
Location: /home/ichard26/dev/oss/pip/venv/lib/python3.12/site-packages
Editable project location: /home/ichard26/dev/oss/pip
Requires: 
Required-by: flit
Metadata-Version: 2.2
Installer: pip
Classifiers:
  [... a bunch of classifiers]
Entry-points:
  [console_scripts]
  pip = pip._internal.cli.main:main
  pip3 = pip._internal.cli.main:main
Project-URLs:
  Homepage, https://pip.pypa.io/
  Documentation, https://pip.pypa.io
  Source, https://github.com/pypa/pip
  Changelog, https://pip.pypa.io/en/stable/news/