Reword "crate not found" resolve message

```
error[E0432]: unresolved import `some_novel_crate`
 --> file.rs:1:5
  |
1 | use some_novel_crate::Type;
  |     ^^^^^^^^^^^^^^^^ use of unresolved module or unlinked crate `some_novel_crate`
```

On resolve errors where there might be a missing crate, mention `cargo add foo`:

```
error[E0433]: failed to resolve: use of unresolved module or unlinked crate `nope`
  --> $DIR/conflicting-impl-with-err.rs:4:11
   |
LL | impl From<nope::Thing> for Error {
   |           ^^^^ use of unresolved module or unlinked crate `nope`
   |
   = help: if you wanted to use a crate named `nope`, use `cargo add nope` to add it to your `Cargo.toml`
```

Author: estebank

tests/fail/rustc-error2.rs

@@ -4,7 +4,7 @@ struct Struct<T>(T);
 impl<T> std::ops::Deref for Struct<T> {
     type Target = dyn Fn(T);
     fn deref(&self) -> &assert_mem_uninitialized_valid::Target {
-        //~^ERROR: undeclared crate or module
+        //~^ERROR: use of unresolved module or unlinked crate
         unimplemented!()
     }
 }

tests/fail/rustc-error2.stderr

@@ -1,8 +1,10 @@
-error[E0433]: failed to resolve: use of undeclared crate or module `assert_mem_uninitialized_valid`
+error[E0433]: failed to resolve: use of unresolved module or unlinked crate `assert_mem_uninitialized_valid`
   --> tests/fail/rustc-error2.rs:LL:CC
    |
 LL |     fn deref(&self) -> &assert_mem_uninitialized_valid::Target {
-   |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ use of undeclared crate or module `assert_mem_uninitialized_valid`
+   |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ use of unresolved module or unlinked crate `assert_mem_uninitialized_valid`
+   |
+   = help: you might be missing a crate named `assert_mem_uninitialized_valid`
 
 error: aborting due to 1 previous error