Add possibility to go through vendor namespaces also

[fogrye]

This issue is about adding, at least optionally, possibility for GlobClassExporter to go through vendor dependencies.

I had a discussion in other project which uses this lib and we agreed that this is what makes sense to have. I get that it will be a little slower but should not be as bad - go through required dependencies, read theirs composer.json, if autoloader namespace matches - process also them.

I would happily contribute to this one, I just want to know from maintainers that this change is ok for them and pr won't be canceled.