internal/pkg: encode entry in custom format

The fact that Gob serialisation is deterministic is an implementation detail. This change replaces Gob with a simple custom format.

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

internal/pkg/pkg_test.go

@@ -211,7 +211,7 @@ func TestCache(t *testing.T) {
 			}
 		}, func(t *testing.T, base *check.Absolute) {
 			wantChecksum := pkg.MustDecode(
-				"lvK4lY9bQUFscHpxqHmiPvptjUwOgn3BFhzCXZMeupkY1n22WUPSuh7pswEvVZrx",
+				"ZNSQH-mjhtIbFvi51lQ0UjatjoS8_5ILrBPNWlO2LWTq9P6MJEnekYzP0esUJnVr",
 			)
 			if checksum, err := pkg.HashDir(base); err != nil {
 				t.Fatalf("HashDir: error = %v", err)