No, it's not a bad tone to keep windrow in your package, even if your bundle via composer.
It all depends on the problems which need to be addressed.
As an example, symfony/phpunit-bridge where the package, when you start depending on php version and other parameters ustanavlivaet in its directory certain compatible version of phpunit and runs...
And a bunch of other packages that store vendors.
I feel that the module can be set via admise
I understand that loads some files in the admin? If Yes, then smello use
and distribute the package in this way.
The only thing that will need to consider is whether to use the composer autoloader or its wrapper, as not only your package can work with a composer.