I need to know which npm package would allow me to convert all html templates to $templateCache and work with webpack 4

You're not using webpack for packaging your reusable directives and services, are you ?
If you do, that's not what you want to be doing. If you're using TS, use TSC to compile TS to JS (es6 is okay) and publish those js and d.ts files to npm.
Next import whatever you need in your application and let the application do the bundling.