internal/rosa: helpers for common url formats

This cleans up call site of NewPackage.

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

internal/rosa/elfutils.go

@@ -7,10 +7,10 @@ func (t Toolchain) newElfutils() (pkg.Artifact, string) {
 		version  = "0.194"
 		checksum = "Q3XUygUPv9vR1TkWucwUsQ8Kb1_F6gzk-KMPELr3cC_4AcTrprhVPMvN0CKkiYRa"
 	)
-	return t.NewPackage("elfutils", version, pkg.NewHTTPGetTar(
-		nil, "https://sourceware.org/elfutils/ftp/"+
+	return t.NewPackage("elfutils", version, newTar(
+		"https://sourceware.org/elfutils/ftp/"+
 			version+"/elfutils-"+version+".tar.bz2",
-		mustDecode(checksum),
+		checksum,
 		pkg.TarBzip2,
 	), &PackageAttr{
 		Env: []string{