Point at unit structs in type errors when they are the pattern of a let binding

[estebank]

We currently already do this when the unit struct is from the current crate:

error[E0308]: mismatched types
 --> fi.rs:8:9
  |
1 | struct percentage;
  | ----------------- unit struct defined here
...
8 |     let percentage = 4i32;
  |         ^^^^^^^^^^   ---- this expression has type `i32`
  |         |
  |         expected `i32`, found `percentage`
  |         `percentage` is interpreted as a unit struct, not a new binding
  |         help: introduce a new binding instead: `other_percentage`

but we must do that also when they come from other crates as well.

https://hachyderm.io/@LGUG2Z/113052067582966834