internal/rosa: helpers for common url formats

This cleans up call site of NewPackage.

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

internal/rosa/xz.go

@@ -7,10 +7,11 @@ func (t Toolchain) newXZ() (pkg.Artifact, string) {
 		version  = "5.8.3"
 		checksum = "nCdayphPGdIdVoAZ2hR4vYlhDG9LeVKho_i7ealTud4Vxy5o5dWe0VwFlN7utuUL"
 	)
-	return t.NewPackage("xz", version, pkg.NewHTTPGetTar(
-		nil, "https://github.com/tukaani-project/xz/releases/download/"+
-			"v"+version+"/xz-"+version+".tar.bz2",
-		mustDecode(checksum),
+	return t.NewPackage("xz", version, newFromGitHubRelease(
+		"tukaani-project/xz",
+		"v"+version,
+		"xz-"+version+".tar.bz2",
+		checksum,
 		pkg.TarBzip2,
 	), nil, (*MakeHelper)(nil),
 		Diffutils,