internal/rosa: helpers for common url formats

This cleans up call site of NewPackage.

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

internal/rosa/qemu.go

@@ -7,9 +7,9 @@ func (t Toolchain) newQEMU() (pkg.Artifact, string) {
 		version  = "10.2.2"
 		checksum = "uNzRxlrVoLWe-EmZmBp75SezymgE512iE5XN90Bl7wi6CjE_oQGQB-9ocs7E16QG"
 	)
-	return t.NewPackage("qemu", version, pkg.NewHTTPGetTar(
-		nil, "https://download.qemu.org/qemu-"+version+".tar.bz2",
-		mustDecode(checksum),
+	return t.NewPackage("qemu", version, newTar(
+		"https://download.qemu.org/qemu-"+version+".tar.bz2",
+		checksum,
 		pkg.TarBzip2,
 	), &PackageAttr{
 		Patches: []KV{