internal/rosa: helpers for common url formats

This cleans up call site of NewPackage.

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

internal/rosa/libcap.go

@@ -7,10 +7,10 @@ func (t Toolchain) newLibcap() (pkg.Artifact, string) {
 		version  = "2.78"
 		checksum = "wFdUkBhFMD9InPnrBZyegWrlPSAg_9JiTBC-eSFyWWlmbzL2qjh2mKxr9Kx2a8ut"
 	)
-	return t.NewPackage("libcap", version, pkg.NewHTTPGetTar(
-		nil, "https://git.kernel.org/pub/scm/libs/libcap/libcap.git/"+
+	return t.NewPackage("libcap", version, newTar(
+		"https://git.kernel.org/pub/scm/libs/libcap/libcap.git/"+
 			"snapshot/libcap-"+version+".tar.gz",
-		mustDecode(checksum),
+		checksum,
 		pkg.TarGzip,
 	), &PackageAttr{
 		// uses source tree as scratch space