Import PEP 518 specification

[jeanas]

This relates to #955 and #1093.

I am aware of the previous attempt in #1111. This PR has smaller commits and does less. (The integration could be much better, but let's please not be perfectionistic and get the text there first; we can improve in subsequent PRs.)

@pradyunsg @pfmoore @brettcannon @CAM-Gerlach

[jeanas]

By the way, I have a question: do we want to keep the JSON-schema structure?

It's presented as “for illustrative purposes only”, and it would need to be adapted when we will integrate PEP 517. I'm not sure it's worth keeping.

[pfmoore]

By the way, I have a question: do we want to keep the JSON-schema structure?

I have no personal use for it. But some people might. I'd keep it, but don't worry about updating it. If it bothers you, add a note saying it reflectes the initial definition from PEP 518 and has not been updated to incorporate later specifications, and leave it at that.

A direct copy of PEP 518 with flaws that can be fixed later by interested parties is better than a PR that tries to describe the current situation accurately but doesn't get merged 🙂

[jeanas]

OK, I'll leave it.

[pradyunsg]

Thanks @jeanas! ^>^