How to split functions.ts into multiple files and still use webpack plugin CustomFunctionsMetadataPlugin

My excel addin project (generated by yeoman - Typescript) uses a webpack plugin "CustomFunctionsMetadataPlugin" to generate the functions.json file.

Snippet from webpack.config.js

      new CustomFunctionsMetadataPlugin({
        output: "functions.json",
        input: "./src/functions/functions.ts"
      }),

We are creating many custom functions and so for development purposes we need to split the functions.ts file up into multiple files. What's the best way to do this yet still have "CustomFunctionsMetadataPlugin" generate the required functions.json file?

Cheers