Include references to the proto, fmt, and math packages, consistent with
the ones in the previous generator:
// Reference imports to suppress errors if they are not otherwise used.
var _ = proto.Marshal
var _ = fmt.Errorf
var _ = math.Inf
Sort imports with ast.SortImports to ensure gofmt-compatible output.
Copy the protoc-gen-go/testdata/imports test files from
github.com/golang/protobuf.
Change-Id: I263e2ac750d03596ae9cb23609049d5ef1016205
Reviewed-on: https://go-review.googlesource.com/135361
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
