internal/rosa: helpers for common url formats

This cleans up call site of NewPackage.

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

internal/rosa/meson.go

@@ -23,10 +23,11 @@ python3 setup.py \
 	install \
 	--prefix=/system \
 	--root=/work
-`, pkg.Path(AbsUsrSrc.Append("meson"), true, pkg.NewHTTPGetTar(
-		nil, "https://github.com/mesonbuild/meson/releases/download/"+
-			version+"/meson-"+version+".tar.gz",
-		mustDecode(checksum),
+`, pkg.Path(AbsUsrSrc.Append("meson"), true, newFromGitHubRelease(
+		"mesonbuild/meson",
+		version,
+		"meson-"+version+".tar.gz",
+		checksum,
 		pkg.TarGzip,
 	))), version
 }