Request: always import from `core` when possible

[Lokathor]

Now that we no longer need to use extern crate core to use paths into the core crate, I would like an RA setting to always select the core path for an item whenever possible. For example, if importing Debug, prefer core::fmt::Debug to std::fmt::Debug.

I often want to develop crates that are fundamentally no_std, but use printing during debugging, so I don't actually turn on no_std until the moment of publishing, at which point I have to fix any imports that RA made. If RA could just always prefer core this would help me.

[lnicola]

I thought we actually had some form of auto-detection here. It would be nice to automatically use core if the current file already has imports from there.