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/resolvercore_test.go

@@ -29,7 +29,7 @@ func typecheckForSystemResolver(t *testing.T, resolver model.Resolver, logger mo
 }
 
 func TestNewResolverSystem(t *testing.T) {
-	resolver := NewResolverStdlib(model.DiscardLogger)
+	resolver := NewStdlibResolver(model.DiscardLogger)
 	typecheckForSystemResolver(t, resolver, model.DiscardLogger)
 }