Show hint to import unqualified types/values

For example:
```gleam
import gleam/dict

type Grid(a) {
  Grid(Dict(#(Int, Int), a))
}
```
This is an error, since `Dict` is not imported. However, `gleam/dict` is. So we could either give a hint to import it unqualified: `gleam/dict.{type Dict}`, or (probably preferably), qualify it: `dict.Dict(...)`
We could either just check for this in imported modules, or all importable modules.