Refactor GPG signer/verifier

Goal is to make it easier to plug in another implementation.

api/repos.go

@@ -9,6 +9,7 @@ import (
 	"github.com/smira/aptly/aptly"
 	"github.com/smira/aptly/database"
 	"github.com/smira/aptly/deb"
+	"github.com/smira/aptly/pgp"
 	"github.com/smira/aptly/utils"
 )
 
@@ -296,7 +297,7 @@ func apiReposPackageFromDir(c *gin.Context) {
 		return
 	}
 
-	verifier := &utils.GpgVerifier{}
+	verifier := &pgp.GpgVerifier{}
 
 	var (
 		sources                      []string