Add subdirectory to Direct URL schema

[konstin]

The Direct URL JSON schema was missing an entry for subdirectory as described in the text.

I've also updated the schema to the 2020-12 draft, which removed some false positives about the schema in my URL.

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📚 Documentation preview 📚: https://python-packaging-user-guide--1797.org.readthedocs.build/en/1797/