internal/pkg: stream decompress artifact

The tarArtifact predates FileArtifact pipelining. This migrates decompression and buffering into a standalone artifact implementation.

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

internal/pkg/pkg_test.go

@@ -293,15 +293,15 @@ func TestIdent(t *testing.T) {
 		a    pkg.Artifact
 		want unique.Handle[pkg.ID]
 	}{
-		{"tar", &stubArtifact{
-			pkg.KindTar,
-			[]byte{pkg.TarGzip, 0, 0, 0, 0, 0, 0, 0},
+		{"decompress", &stubArtifact{
+			pkg.KindDecompress,
+			[]byte{pkg.Gzip, 0, 0, 0, 0, 0, 0, 0},
 			[]pkg.Artifact{
 				overrideIdent{pkg.ID{}, new(stubArtifact)},
 			},
 			nil,
 		}, unique.Make[pkg.ID](pkg.MustDecode(
-			"WKErnjTOVbuH2P9a0gM4OcAAO4p-CoX2HQu7CbZrg8ZOzApvWoO3-ISzPw6av_rN",
+			"97Y85QewssfPbNIN9cyNhzD4e6dLHcDTU8rb2c34k-aCrZfBNXFUc0duPiLFFcw_",
 		))},
 	}