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

[gazerro]

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"

[ianlancetaylor]

This is not an API change and does not need to be a proposal. Taking it out of the proposal process.

[gopherbot]

Change https://golang.org/cl/334989 mentions this issue: cmd/compile, go/types, types2: rename 'imported and not used' error to 'imported but not used'