Add the ability to reserve a name

[dstufft]

In pypa/packaging.python.org#271 and #1627 we talked about the register api, whether we want to support it or not and what use cases it solves that might not be currently solved. While I decided to continue not to support it for now, their is possibly some desire around a pre-reservation API for names.

What this looks like exactly who knows, but reading the two linked threads can give some background on it.

[brainwane]

We discussed this in a meeting this week. PyPI admins can already use the web interface to prohibit names to stop them from being used, and unreserve/allow previously prohibited names.

This issue is more about giving this feature to end users through the web interface. Perhaps a reservation request would enter a queue for moderators/admins to approve (#3231).

(The workaround for PyPI users right now: uploading an empty package.)

[edmorley]

In previous discussions about use-cases for reserving a name, I've seen the following mentioned:

• Reserving a package name before the release of a new public package, to prevent a race condition between the landing of code in a public GitHub repo and the package actually being published.
• Reserving the name of an internal package, to prevent dependency-confusion type issues if tools/internal package indexes aren't configured correctly.

However I haven't seen this use-case mentioned yet:

• Wanting to reserve namespaces inside the pyproject.toml [tool] table (as required by PEP518) for tooling that isn't distributed on PyPI (eg configuration for deployment tooling, such as buildpacks).