remove dynamic import statements
Thanks @simov and others for your great work!

## Summary
Importing from paths generated at runtime breaks
parcel, see (parcel-bundler/parcel#4031),
or esbuild, see (evanw/esbuild#480).
Building fails or throws a warning like this:
```
warning: This call to "require" will not be bundled because the argument is not a string literal (surround with a try/catch to silence this warning)
    32 │   var handler = require(`./lib/handler/koa-${version}`)
```
This can lead to problems at runtime.