Warn (or error) when setuptools.build_meta:__legacy__ is explicitly specified

Per the original discussion, setuptools.build_meta:__legacy__ was intended only as a default and was not intended to be specified as the build backend. If we're going to go ahead with #1688, we'll need to be able to parse pyproject.toml anyway, so this could be a fairly simple way to start working with pyproject.toml and get started on #1688 without committing to support a whole new format.

I'd say it would be a good idea to get this done sooner rather than later, and preferably just make it an error directly, before too many people start using setuptools.build_meta:__legacy__ directly.

Of course, the major downside to this is that it sorta breaks the separation between front-end and back-end in PEP 517, since we now have to read the front end's configuration to determine if it's being used incorrectly. I'm not sure if it's worth maintaining the purity of this separation at the cost of a bunch of people starting to explicitly depend on setuptools.build_meta:__legacy__.

Start by reading the existing handling of pyproject.toml and the setuptools.build_meta:legacy backend entry point. Determine whether explicit use should warn or error, while considering the PEP 517 front-end/back-end separation and the related work in #1688. Done means the chosen behavior is implemented and covered by the relevant project checks.