Remove the need for both preval & codegen

[jesstelford]

The babel implementation currently requires both babel-plugin-codegen & babel-plugin-preval leading to a verbose 2-line format:

const filename = preval`module.exports = __filename`;
const Layout = codegen.require("@ceteio/next-layout-loader", filename);

Is there a way to remove the need for preval?

When I try to add __filename directly as the second parameter to codegen.require, I get an error:

const Layout = codegen.require("@ceteio/next-layout-loader", __filename);

Error: codegen cannot determine the value of an argument in codegen.require
    at Array.map (<anonymous>)