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>

errdefs/errors.go

@@ -26,7 +26,7 @@ var (
 	ErrAlreadyExists      = errors.New("already exists")
 	ErrFailedPrecondition = errors.New("failed precondition")
 	ErrUnavailable        = errors.New("unavailable")
-	ErrNotSupported       = errors.New("not supported") // represents not supported and unimplemented
+	ErrNotImplemented     = errors.New("not implemented") // represents not supported and unimplemented
 )
 
 func IsInvalidArgument(err error) bool {
@@ -54,6 +54,6 @@ func IsUnavailable(err error) bool {
 	return errors.Cause(err) == ErrUnavailable
 }
 
-func IsNotSupported(err error) bool {
-	return errors.Cause(err) == ErrNotSupported
+func IsNotImplemented(err error) bool {
+	return errors.Cause(err) == ErrNotImplemented
 }

errdefs/grpc.go

@@ -38,7 +38,7 @@ func ToGRPC(err error) error {
 		return grpc.Errorf(codes.FailedPrecondition, err.Error())
 	case IsUnavailable(err):
 		return grpc.Errorf(codes.Unavailable, err.Error())
-	case IsNotSupported(err):
+	case IsNotImplemented(err):
 		return grpc.Errorf(codes.Unimplemented, err.Error())
 	}
 
@@ -72,7 +72,7 @@ func FromGRPC(err error) error {
 	case codes.FailedPrecondition:
 		cls = ErrFailedPrecondition
 	case codes.Unimplemented:
-		cls = ErrNotSupported
+		cls = ErrNotImplemented
 	default:
 		cls = ErrUnknown
 	}