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/ir_test.go

@@ -104,6 +104,8 @@ func TestIRRoundtrip(t *testing.T) {
 
 		{"file anonymous", pkg.NewFile("", []byte{0})},
 		{"file", pkg.NewFile("stub", []byte("stub"))},
+
+		{"decompress", pkg.NewDecompress(pkg.NewFile("", []byte{0}), pkg.Bzip2)},
 	}
 	testCasesCache := make([]cacheTestCase, len(testCases))
 	for i, tc := range testCases {