internal/pkg: optionally named static file

These are generally for generating configuration files or build scripts, naming them is quite useful.

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

internal/pkg/pkg_test.go

@@ -496,7 +496,7 @@ func TestCache(t *testing.T) {
 				if _, _, err = c.Cure(stubArtifactF{
 					kind:   pkg.KindExec,
 					params: []byte("unreachable artifact cured after cancel"),
-					deps:   []pkg.Artifact{pkg.NewFile([]byte("unreachable dependency"))},
+					deps:   []pkg.Artifact{pkg.NewFile("", []byte("unreachable dependency"))},
 				}); !reflect.DeepEqual(err, context.Canceled) {
 					t.Fatalf("(closed) Cure: error = %v", err)
 				}