internal/pkg: archive unpack artifact

This unpacks an internal/pkg archive stream used in the upcoming mirror service.

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

internal/pkg/ir_test.go

@@ -98,6 +98,8 @@ func TestIRRoundtrip(t *testing.T) {
 		{"file", pkg.NewFile("stub", []byte("stub"))},
 
 		{"decompress", pkg.NewDecompress(pkg.NewFile("", []byte{0}), pkg.Bzip2)},
+
+		{"archive", pkg.NewArchive(pkg.NewFile("", []byte{0}))},
 	}
 	testCasesCache := make([]cacheTestCase, len(testCases))
 	for i, tc := range testCases {