How do I do namespacing in a module from an external project which is not in the src directory?
From your post, it looks like you added the above composer.json file, then ran composer from the module root. This is not the way to do it. This will create a vendor folder inside your module folder, which is not what you want.
What you need to do is add the composer.json file to your module, then push that to the remote repository. Then use Composer to install and manage the module. Doing this will add the library to the vendor folder for your project, and the library will be available when calling \ccxt\ccxt