Do not use question as label

We don't want to have questions in the diagnostic output. Instead, we use wording that communicates uncertainty, like "might":

```
error[E0432]: unresolved import `spam`
  --> $DIR/import-from-missing-star-3.rs:2:9
   |
LL |     use spam::*;
   |         ^^^^ you might be missing crate `spam`
   |
   = help: consider adding `extern crate spam` to use the `spam` crate
```

Author: estebank

compiler/rustc_resolve/src/diagnostics.rs

@@ -2012,7 +2012,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                 )
             } else if ident.name == sym::core {
                 (
-                    format!("maybe a missing crate `{ident}`?"),
+                    format!("you might be missing crate `{ident}`"),
                     Some((
                         vec![(ident.span, "std".to_string())],
                         "try using `std` instead of `core`".to_string(),
@@ -2021,7 +2021,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                 )
             } else if self.tcx.sess.is_rust_2015() {
                 (
-                    format!("maybe a missing crate `{ident}`?"),
+                    format!("you might be missing crate `{ident}`"),
                     Some((
                         vec![],
                         format!(

tests/rustdoc-ui/ice-unresolved-import-100241.stderr

@@ -2,7 +2,7 @@ error[E0432]: unresolved import `inner`
   --> $DIR/ice-unresolved-import-100241.rs:9:13
    |
 LL |     pub use inner::S;
-   |             ^^^^^ maybe a missing crate `inner`?
+   |             ^^^^^ you might be missing crate `inner`
    |
    = help: consider adding `extern crate inner` to use the `inner` crate