Warn about missing license field

Doesn't yet *fix* anything, but this is a good start, isn't it?
Do you mean the `license` / `licence` split, or do you mean the `license` / `licenses` split? If the former, then I think leaving it be is for the best. If it's the latter, we probably need to bikeshed a little; I seem to recall `licenses` being deprecated at some point, but it's not at all uncommon for projects to have multiple licenses, and we should support that, even if we don't get drawn into trying to solve the infinite bikeshed of semantic licensing specification (big companies frequently have automated license checkers scanning their OSS projects to make sure they're in compliance; sometimes the systems they come up with to enforce these are terrifyingly convoluted).