internal/rosa: helpers for common url formats

This cleans up call site of NewPackage.

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

internal/rosa/mksh.go

@@ -26,10 +26,9 @@ cp -v lksh /work/system/bin/sh
 
 mkdir -p /work/bin/
 ln -vs ../system/bin/sh /work/bin/
-`, pkg.Path(AbsUsrSrc.Append("mksh"), false, pkg.NewHTTPGetTar(
-		nil,
+`, pkg.Path(AbsUsrSrc.Append("mksh"), false, newTar(
 		"https://mbsd.evolvis.org/MirOS/dist/mir/mksh/mksh-R"+version+".tgz",
-		mustDecode(checksum),
+		checksum,
 		pkg.TarGzip,
 	))), version
 }