internal/rosa: helpers for common url formats

This cleans up call site of NewPackage.

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

internal/rosa/rsync.go

@@ -7,10 +7,10 @@ func (t Toolchain) newRsync() (pkg.Artifact, string) {
 		version  = "3.4.1"
 		checksum = "VBlTsBWd9z3r2-ex7GkWeWxkUc5OrlgDzikAC0pK7ufTjAJ0MbmC_N04oSVTGPiv"
 	)
-	return t.NewPackage("rsync", version, pkg.NewHTTPGetTar(
-		nil, "https://download.samba.org/pub/rsync/src/"+
+	return t.NewPackage("rsync", version, newTar(
+		"https://download.samba.org/pub/rsync/src/"+
 			"rsync-"+version+".tar.gz",
-		mustDecode(checksum),
+		checksum,
 		pkg.TarGzip,
 	), &PackageAttr{
 		Flag: TEarly,