This commit feature gates generating modules and macro definitions in procedural
macro expansions. Custom derive is exempt from this check as it would be a large
retroactive breaking change (#50587). It's hoped that we can hopefully stem the
bleeding to figure out a better solution here before opening up the floodgates.
The restriction here is specifically targeted at surprising hygiene results [1]
that result in non-"copy/paste" behavior. Hygiene and procedural macros is
intended to be avoided as much as possible for Macros 1.2 by saying everything
is "as if you copy/pasted the code", but modules and macros are sort of weird
exceptions to this rule that aren't fully fleshed out.
[1]: https://github.com/rust-lang/rust/issues/50504#issuecomment-387734625
cc #50504
5e4bac31b8