internal/rosa: helpers for common url formats

This cleans up call site of NewPackage.

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

internal/rosa/rdfind.go

@@ -7,9 +7,9 @@ func (t Toolchain) newRdfind() (pkg.Artifact, string) {
 		version  = "1.8.0"
 		checksum = "PoaeJ2WIG6yyfe5VAYZlOdAQiR3mb3WhAUMj2ziTCx_IIEal4640HMJUb4SzU9U3"
 	)
-	return t.NewPackage("rdfind", version, pkg.NewHTTPGetTar(
-		nil, "https://rdfind.pauldreik.se/rdfind-"+version+".tar.gz",
-		mustDecode(checksum),
+	return t.NewPackage("rdfind", version, newTar(
+		"https://rdfind.pauldreik.se/rdfind-"+version+".tar.gz",
+		checksum,
 		pkg.TarGzip,
 	), nil, &MakeHelper{
 		// test suite hard codes /bin/echo