Use Platform instead of generated API

Signed-off-by: Maksym Pavlenko <pavlenko.maksym@gmail.com>

platforms/platforms.go

@@ -114,14 +114,18 @@ import (
 	"strconv"
 	"strings"
 
-	"github.com/containerd/containerd/errdefs"
 	specs "github.com/opencontainers/image-spec/specs-go/v1"
+
+	"github.com/containerd/containerd/errdefs"
 )
 
 var (
 	specifierRe = regexp.MustCompile(`^[A-Za-z0-9_-]+$`)
 )
 
+// Platform is a type alias for convenience, so there is no need to import image-spec package everywhere.
+type Platform = specs.Platform
+
 // Matcher matches platforms specifications, provided by an image or runtime.
 type Matcher interface {
 	Match(platform specs.Platform) bool