This module cannot add a dependency on an AWS library. If this dependency nee...

This module cannot add a dependency on an AWS library. If this dependency needs to be added in order for S3 to work, it might be worth splitting the S3 functionality into a (bundled) submodule that's only available when enabled.

I agree it is not a good idea to require on the main module level; however, I'm not sure if the submodule approach would work. I don't know if a composer.json inside a submodule is read to build the dependencies. I broadly see two approaches:

1. Have the functionality in a submodule and only add the PHP dependency in `suggests` section. The functionality is better off in a submodule. Otherwise, things like extending a class from the PHP package might not work as expected. The module shouldn't be able to install without the dependency.
2. Put the entire thing in Backup and Migrate S3 module. Over there, it might make sense to put the dependency in the root composer.json itself.

Thoughts?