separate proc interfaces into standalone package

Signed-off-by: Evan Hazlett <ejhazlett@gmail.com>

runtime/linux/proc/deleted_state.go

@@ -22,6 +22,7 @@ import (
 	"context"
 
 	"github.com/containerd/console"
+	"github.com/containerd/containerd/runtime/proc"
 	google_protobuf "github.com/gogo/protobuf/types"
 	"github.com/pkg/errors"
 )
@@ -65,6 +66,6 @@ func (s *deletedState) SetExited(status int) {
 	// no op
 }
 
-func (s *deletedState) Exec(ctx context.Context, path string, r *ExecConfig) (Process, error) {
+func (s *deletedState) Exec(ctx context.Context, path string, r *ExecConfig) (proc.Process, error) {
 	return nil, errors.Errorf("cannot exec in a deleted state")
 }