internal/rosa: helpers for common url formats

This cleans up call site of NewPackage.

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

internal/rosa/tamago.go

@@ -31,11 +31,10 @@ rm \
 	os/root_unix_test.go
 
 ./all.bash
-`, pkg.Path(AbsUsrSrc.Append("tamago"), false, pkg.NewHTTPGetTar(
-		nil, "https://github.com/usbarmory/tamago-go/archive/refs/tags/"+
-			"tamago-go"+version+".tar.gz",
-		mustDecode(checksum),
-		pkg.TarGzip,
+`, pkg.Path(AbsUsrSrc.Append("tamago"), false, newFromGitHub(
+		"usbarmory/tamago-go",
+		"tamago-go"+version,
+		checksum,
 	))), version
 }
 func init() {