Remove extra package.

Signed-off-by: David Calavera <david.calavera@gmail.com>

osxkeychain/cmd/main_darwin.go

@@ -1,10 +1,10 @@
 package main
 
 import (
+	"github.com/calavera/docker-credential-helpers/credentials"
 	"github.com/calavera/docker-credential-helpers/osxkeychain"
-	"github.com/calavera/docker-credential-helpers/plugin"
 )
 
 func main() {
-	plugin.Serve(osxkeychain.New())
+	credentials.Serve(osxkeychain.New())
 }

osxkeychain/osxkeychain_darwin.go

@@ -89,7 +89,7 @@ func (h osxkeychain) Get(serverURL string) (string, string, error) {
 		goMsg := C.GoString(errMsg)
 
 		if goMsg == notFoundError {
-			return "", "", credentials.NotFoundError
+			return "", "", credentials.ErrCredentialsNotFound
 		}
 
 		return "", "", errors.New(goMsg)