Prefer std paths to core/alloc paths in autoimport etc.

[flodiebold]

Autoimport and path qualification should prefer to use std paths before core or alloc. I think this probably requires a small hack here to hardcode this preference; at least I can't think of any principled rule for this.

(Note that this is unrelated to no_std; once we have no_std support, the path finding algorithm should automatically only find core paths in such crates.)

[SomeoneToIgnore]

Does it make any sense to add core and alloc entries here: https://github.com/rust-analyzer/rust-analyzer/blob/master/crates/ra_hir_expand/src/name.rs#L142 to be able to compare them as names in the line proposed in the ticket?

[flodiebold]

Probably, yeah!