internal/rosa: helpers for common url formats

This cleans up call site of NewPackage.

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

internal/rosa/libgd.go

@@ -7,10 +7,10 @@ func (t Toolchain) newLibgd() (pkg.Artifact, string) {
 		version  = "2.3.3"
 		checksum = "8T-sh1_FJT9K9aajgxzh8ot6vWIF-xxjcKAHvTak9MgGUcsFfzP8cAvvv44u2r36"
 	)
-	return t.NewPackage("libgd", version, pkg.NewHTTPGetTar(
-		nil, "https://github.com/libgd/libgd/releases/download/"+
-			"gd-"+version+"/libgd-"+version+".tar.gz",
-		mustDecode(checksum),
+	return t.NewPackage("libgd", version, newFromGitHubRelease(
+		"libgd/libgd",
+		"gd-"+version,
+		"libgd-"+version+".tar.gz", checksum,
 		pkg.TarGzip,
 	), &PackageAttr{
 		Env: []string{