when (compiles do: import a): is very broken

[timothee]

> A builtin canImport looks much more useful.

Actually the one I'd like to implement is `whichModule` 
(<https://github.com/timotheecour/Nim/issues/376>) which subsumes `canImport`.
    
    
    const path = whichModule("exitprocs")
    
    
    Run

(from which `canImport` is easy to implement, simply via 
`whichModule("exitprocs").len > 0`)

There are valid cases, and the workarounds without it are not good.