Canonicalize fullname, honoring PEP 625

[jaraco]

Ref pypa/setuptools#3593 and pypa/setuptools#4286.

• Support PEP 625
• Expect to find canonicalize_ functions in packaging.*
• Update tests to match new expectation.
• Fix canonicalization
• In test_sdist, provide a more complex name to capture canonicalization behavior.
• Add packaging as a vendored package.
• Use vendored packaging.