Module of PsiElement in dependency

Hello everyone,

I'm developing a custom language plugin and I am stuck resolving references across dependencies.

The test project has a dependency to another project, and references an element of this dependency.
This works, however, the resolve in the read-only file of the dependency does not work.

For project files the following works:

ProjectRootManager.getInstance (element.getProject()) .getFileIndex () .getModuleForFile (element.getContainingFile ().getVirtualFile ());

However, if element is an PsiElement inside a dependency, then the module is null.
How can I get the module content of such an element?