internal/rosa: helpers for common url formats

This cleans up call site of NewPackage.

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

internal/rosa/argp-standalone.go

@@ -7,10 +7,10 @@ func (t Toolchain) newArgpStandalone() (pkg.Artifact, string) {
 		version  = "1.3"
 		checksum = "vtW0VyO2pJ-hPyYmDI2zwSLS8QL0sPAUKC1t3zNYbwN2TmsaE-fADhaVtNd3eNFl"
 	)
-	return t.NewPackage("argp-standalone", version, pkg.NewHTTPGetTar(
-		nil, "http://www.lysator.liu.se/~nisse/misc/"+
+	return t.NewPackage("argp-standalone", version, newTar(
+		"http://www.lysator.liu.se/~nisse/misc/"+
 			"argp-standalone-"+version+".tar.gz",
-		mustDecode(checksum),
+		checksum,
 		pkg.TarGzip,
 	), &PackageAttr{
 		Env: []string{