internal/pkg: append user-facing name in messages

This makes verbose messages much more useful.

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

internal/pkg/tar.go

@@ -6,6 +6,7 @@ import (
 	"compress/gzip"
 	"encoding/binary"
 	"errors"
+	"fmt"
 	"io"
 	"io/fs"
 	"net/http"
@@ -32,11 +33,29 @@ type tarArtifact struct {
 	compression uint64
 }
 
+// tarArtifactNamed embeds tarArtifact for a [fmt.Stringer] tarball.
+type tarArtifactNamed struct {
+	tarArtifact
+	// Copied from tarArtifact.f.
+	name string
+}
+
+var _ fmt.Stringer = new(tarArtifactNamed)
+
+// String returns the name of the underlying [Artifact] suffixed with unpack.
+func (a *tarArtifactNamed) String() string { return a.name + "-unpack" }
+
 // NewTar returns a new [Artifact] backed by the supplied [Artifact] and
 // compression method. The source [Artifact] must be compatible with
 // [TContext.Open].
 func NewTar(a Artifact, compression uint64) Artifact {
-	return &tarArtifact{a, compression}
+	ta := tarArtifact{a, compression}
+	if s, ok := a.(fmt.Stringer); ok {
+		if name := s.String(); name != "" {
+			return &tarArtifactNamed{ta, name}
+		}
+	}
+	return &ta
 }
 
 // NewHTTPGetTar is abbreviation for NewHTTPGet passed to NewTar.