internal/rosa: helpers for common url formats

This cleans up call site of NewPackage.

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

internal/rosa/ncurses.go

@@ -7,9 +7,9 @@ func (t Toolchain) newNcurses() (pkg.Artifact, string) {
 		version  = "6.6"
 		checksum = "XvWp4xi6hR_hH8XUoGY26L_pqBSDapJYulhzZqPuR0KNklqypqNc1yNXU-nOjf5w"
 	)
-	return t.NewPackage("ncurses", version, pkg.NewHTTPGetTar(
-		nil, "https://ftpmirror.gnu.org/gnu/ncurses/ncurses-"+version+".tar.gz",
-		mustDecode(checksum),
+	return t.NewPackage("ncurses", version, newTar(
+		"https://ftpmirror.gnu.org/gnu/ncurses/ncurses-"+version+".tar.gz",
+		checksum,
 		pkg.TarGzip,
 	), nil, &MakeHelper{
 		// "tests" are actual demo programs, not a test suite.