`use foo::self` should only give one error

[nrc]

It currently gives two - one for self outside {} and one for an unknown import. The latter should be silenced.

e.g.,

error[E0429]: `self` imports are only allowed within a { } list
  --> src/librustc_save_analysis/external_data.rs:19:5
   |
19 | use data::self;
   |     ^^^^^^^^^^

error[E0432]: unresolved import `data::self`
  --> src/librustc_save_analysis/external_data.rs:19:5
   |
19 | use data::self;
   |     ^^^^^^^^^^ no `self` in `data`

[tommyip]

I will work on this

[tommyip]

On an related note, shouldn't there be a lint for use foo::{self}. Maybe say "you can just use use foo."