cleanup: netx does not use netxlite legacy names (#801)

This diff refactors netx and netxlite to ensure we're not using
netxlite legacy names inside of netx.

To this end, we're cheating a bit. We're exposing a new factory to
get an unwrapped stdlib resolver rather than defining a legacy name
to export the private name of the same factory.

This is actually a fine place to stop, for now, the next and
netxlite refactoring at https://github.com/ooni/probe/issues/2121.

internal/netxlite/http_test.go

@@ -236,7 +236,7 @@ func TestNewHTTPTransport(t *testing.T) {
 					called.Add(1)
 				},
 			},
-			Resolver: NewResolverStdlib(log.Log),
+			Resolver: NewStdlibResolver(log.Log),
 		}
 		td := NewTLSDialer(d, NewTLSHandshakerStdlib(log.Log))
 		txp := NewHTTPTransport(log.Log, d, td)