rustpkg invokes rustc without going through `rustc::monitor`

[jld]

This may be a bigger problem, but one concrete effect (seen in #9613) is that rustpkg doesn't set the larger-than-default stack size that rustc would normally get. (The spawn responsible for that is actually in extra::workcache, not rustpkg itself, and it doesn't take a task builder — if I edit workcache to set a stack size then the tests pass but that's probably not the right solution.)

[catamorphism]

I've implemented this change, but I don't know how to test it. I thought at first I would start using the diagnostic emitter to print out errors in rustpkg, and then a test could check the error message to make sure it was formatted right (since the demitter can only come from monitor). But, the @Emitter is non-sendable, and rustpkg does everything in tasks, so that won't work.

I'm just going to submit the PR without a test.

[catamorphism]

This is actually kind of annoying -- see my comment on #10342

[thestinger]

rustpkg is gone