internal/rosa: helpers for common url formats

This cleans up call site of NewPackage.

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

internal/rosa/libev.go

@@ -7,9 +7,9 @@ func (t Toolchain) newLibev() (pkg.Artifact, string) {
 		version  = "4.33"
 		checksum = "774eSXV_4k8PySRprUDChbEwsw-kzjIFnJ3MpNOl5zDpamBRvC3BqPyRxvkwcL6_"
 	)
-	return t.NewPackage("libev", version, pkg.NewHTTPGetTar(
-		nil, "https://dist.schmorp.de/libev/Attic/libev-"+version+".tar.gz",
-		mustDecode(checksum),
+	return t.NewPackage("libev", version, newTar(
+		"https://dist.schmorp.de/libev/Attic/libev-"+version+".tar.gz",
+		checksum,
 		pkg.TarGzip,
 	), nil, (*MakeHelper)(nil)), version
 }