internal/rosa: helpers for common url formats

This cleans up call site of NewPackage.

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

internal/rosa/flex.go

@@ -9,10 +9,11 @@ func (t Toolchain) newFlex() (pkg.Artifact, string) {
 		version  = "2.6.4"
 		checksum = "p9POjQU7VhgOf3x5iFro8fjhy0NOanvA7CTeuWS_veSNgCixIJshTrWVkc5XLZkB"
 	)
-	return t.NewPackage("flex", version, pkg.NewHTTPGetTar(
-		nil, "https://github.com/westes/flex/releases/download/"+
-			"v"+version+"/flex-"+version+".tar.gz",
-		mustDecode(checksum),
+	return t.NewPackage("flex", version, newFromGitHubRelease(
+		"westes/flex",
+		"v"+version,
+		"flex-"+version+".tar.gz",
+		checksum,
 		pkg.TarGzip,
 	), nil, (*MakeHelper)(nil),
 		M4,