Cabal - Expose all modules while building library

You have to list all modules in the cabal configuration file. In your case, you just put the list of modules after exposed-modules:. There is no simpler way to write a list of modules.

Cabal cannot automatically find the files that are part of an executable or library, so it relies on the list of modules in the configuration file. Unlike GHC, cabal cannot find modules based on import statements in the source code. If you don't list every module, then you may be able to build the project (because GHC can find source files), but other commands such as cabal sdist will not access the source files that aren't listed.

The modern answer is stack + hpack instead of using explicit cabal config. It could automatically expose package modules and provides many other enhancements.