internal/pkg: return reader for files

This improves efficiency for cache hits.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/pkg_test.go

@@ -8,6 +8,7 @@ import (
 	"encoding/binary"
 	"errors"
 	"fmt"
+	"io"
 	"io/fs"
 	"net/http"
 	"os"
@@ -536,10 +537,14 @@ func TestCache(t *testing.T) {
 					fs.ModeDir | 0500,
 				)},
 
-				{"loadData directory", overrideIdent{pkg.ID{0xff, 3}, stubArtifact{
+				{"openFile directory", overrideIdent{pkg.ID{0xff, 3}, stubArtifact{
 					kind: pkg.KindTar,
 					cure: func(c *pkg.CureContext) error {
-						_, err := c.LoadData(overrideChecksumFile{checksum: wantChecksum})
+						r, err := c.OpenFile(overrideChecksumFile{checksum: wantChecksum})
+						if err != nil {
+							panic(err)
+						}
+						_, err = io.ReadAll(r)
 						return err
 					},
 				}}, nil, pkg.Checksum{}, &os.PathError{