internal/rosa: helpers for common url formats

This cleans up call site of NewPackage.

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

internal/rosa/llvm.go

@@ -164,11 +164,10 @@ ln -s ld.lld /work/system/bin/ld
 		}...)
 	}
 
-	return t.NewPackage("llvm", llvmVersion, pkg.NewHTTPGetTar(
-		nil, "https://github.com/llvm/llvm-project/archive/refs/tags/"+
-			"llvmorg-"+llvmVersion+".tar.gz",
-		mustDecode(llvmChecksum),
-		pkg.TarGzip,
+	return t.NewPackage("llvm", llvmVersion, newFromGitHub(
+		"llvm/llvm-project",
+		"llvmorg-"+llvmVersion,
+		llvmChecksum,
 	), &PackageAttr{
 		Patches: slices.Concat(attr.patches, []KV{
 			{"increase-stack-size-unconditional", `diff --git a/llvm/lib/Support/Threading.cpp b/llvm/lib/Support/Threading.cpp