Indeed as other stated, UML does not support directly what you need. As far as I understand, you want capture the structure (modules), behaviour (features) and interactions (dependencies), although in a basic form, still you aim to capture too much at the same time.
So let us review your possibilities with UML, I hope this will help you decide:
Package diagram can show structure and dependencies as packages and imports
Component diagram can capture structure and dependencies as components with connectors
Composite Structure diagram allow to depict complex structure and interactions through composites, ports, connectors and collaborations
Use case diagram contains actors interacting with the system to perform use cases, which can be combined with the package diagram to simply decompose system in different modules, which provide different features - use cases and for depicting interactions you can connect actors with use cases.
So, as you guess my choice will be the use case diagram with package diagram. Those are goth quite simple, containing small amount of noise, which might be essential for your simple case. This combination allows for most of what you need, however, to show that modules interact in certain way, you would need to have a module both as a package and as an actor, which is not a good practice. But from what you say I guess you want to advertise change of existing system, which will look nice in those diagrams and will be easy to explain to customers who know nothing about UML. You might consider removing interactions between modules and show only features provided to users (depicted as actors).