cmd/compile, go/types, types2: rename 'imported and not used' error to 'imported but not used'

Commit 449b6ab has reworded, in Go 1.14, the "declared **and** not used" error message as "declared **but** not used". I propose to do the same for the "imported and not used" error message.

If an imported package is not used, instead of

```
imported and not used: "fmt"
```

the error message would be

```
imported but not used: "fmt"
```

