platforms: implement matcher support

Matching support is now implemented in the platforms package. The
`Parse` function now returns a matcher object that can be used to
match OCI platform specifications. We define this as an interface to
allow the creation of helpers oriented around platform selection.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

services/diff/service.go

@@ -74,7 +74,7 @@ func (s *service) Apply(ctx context.Context, er *diffapi.ApplyRequest) (*diffapi
 
 	for _, differ := range s.differs {
 		ocidesc, err = differ.Apply(ctx, desc, mounts)
-		if !errdefs.IsNotSupported(err) {
+		if !errdefs.IsNotImplemented(err) {
 			break
 		}
 	}
@@ -99,7 +99,7 @@ func (s *service) Diff(ctx context.Context, dr *diffapi.DiffRequest) (*diffapi.D
 
 	for _, differ := range s.differs {
 		ocidesc, err = differ.DiffMounts(ctx, aMounts, bMounts, dr.MediaType, dr.Ref)
-		if !errdefs.IsNotSupported(err) {
+		if !errdefs.IsNotImplemented(err) {
 			break
 		}
 	}