update platforms Parse to return platform spec, decouple matcher

Signed-off-by: Jess Valarezo <valarezo.jessica@gmail.com>

images/handlers.go

@@ -193,10 +193,11 @@ func FilterPlatform(platform string, f HandlerFunc) HandlerFunc {
 
 		var descs []ocispec.Descriptor
 		if platform != "" && isMultiPlatform(desc.MediaType) {
-			matcher, err := platforms.Parse(platform)
+			p, err := platforms.Parse(platform)
 			if err != nil {
 				return nil, err
 			}
+			matcher := platforms.NewMatcher(p)
 
 			for _, d := range children {
 				if d.Platform == nil || matcher.Match(*d.Platform) {

images/image.go

@@ -131,13 +131,13 @@ func Manifest(ctx context.Context, provider content.Provider, image ocispec.Desc
 	var (
 		matcher platforms.Matcher
 		m       *ocispec.Manifest
-		err     error
 	)
 	if platform != "" {
-		matcher, err = platforms.Parse(platform)
+		p, err := platforms.Parse(platform)
 		if err != nil {
 			return ocispec.Manifest{}, err
 		}
+		matcher = platforms.NewMatcher(p)
 	}
 
 	if err := Walk(ctx, HandlerFunc(func(ctx context.Context, desc ocispec.Descriptor) ([]ocispec.Descriptor, error) {