Since Webpack is running in build-time, it can’t figure out which modules to bundle when the name is a dynamic variable. You can give it hints by specifying part of the path (for example, if you know all the modules are in a single directory).
This answer can help:
https://stackoverflow.com/a/33048000
(Also check require.context by Webpack. Another example is karma tests, here.)
Alternatively – if you know the filenames in advanced, it’s better to add another build step to output them a strings to the file, that way Webpack can bundle them.