Tidy should detect unreachable modules in `compiler/`

[compiler-errors]

We shouldn't probably ever have unreachable modules in compiler/, so we should implement a tidy lint against them.

Would've probably caught some super stupid module file duplications that happened due to a failed module rename (twice!!) in 3b9daac, resulting in the clean-up PR: #106205

[clubby789]

@rustbot label +T-bootstrap

[Mark-Simulacrum]

Ultimately a fix here is very likely to look like #28711, which is a long-standing bug. I'm not sure scoping to just compiler/ helps here.

I think it makes sense to close this in favor of that issue since any implementation ought to at least aim to address the full scope, even if it falls short (as many tidy lints do).