internal/rosa: helpers for common url formats

This cleans up call site of NewPackage.

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

internal/rosa/nss.go

@@ -14,11 +14,10 @@ func (t Toolchain) newNSS() (pkg.Artifact, string) {
 		version0  = "4_38_2"
 		checksum0 = "25x2uJeQnOHIiq_zj17b4sYqKgeoU8-IsySUptoPcdHZ52PohFZfGuIisBreWzx0"
 	)
-	return t.NewPackage("nss", version, pkg.NewHTTPGetTar(
-		nil, "https://github.com/nss-dev/nss/archive/refs/tags/"+
-			"NSS_"+strings.Join(strings.SplitN(version, ".", 2), "_")+"_RTM.tar.gz",
-		mustDecode(checksum),
-		pkg.TarGzip,
+	return t.NewPackage("nss", version, newFromGitHub(
+		"nss-dev/nss",
+		"NSS_"+strings.Join(strings.SplitN(version, ".", 2), "_")+"_RTM",
+		checksum,
 	), &PackageAttr{
 		Paths: []pkg.ExecPath{
 			pkg.Path(AbsUsrSrc.Append("nspr.zip"), false, pkg.NewHTTPGet(