Warn if packages still using flit.buildapi instead of flit_core.buildapi

[takluyver]

I'm not 100% sold on this: the removal would break things that might otherwise work, and it's a tiny bit of complexity by itself. But any project still specifying flit.buildapi is probably also using old-style metadata ([tool.flit.metadata] table), which 4.0 will likely not support (#673), and that is enough complexity to be worth cleaning up.

• flit.buildapi added in 0.12 (November 2017)
• flit_core.builadpi added in 2.0 (November 2019)
• New-style metadata ([project] from PEP 621) added in 3.2 (March 2021)

Using python -m build, this warning is visible. Using pip wheel (and presumably any other pip command that does a build), it isn't (pip wheel -v shows it). I don't think there's anything we can do to make it more visible, but a warning that only some people see is better than no warning.

First step for #519