ncc incorrectly includes `__dirname` in `.mjs` output

Version 0.29.x (#720) has made it so that if you have a .mjs file as an input, you get a .mjs file as an output. I would like a flag to get back the 0.28.x behavior of outputting it as a .js file that I can run directly with node, even v10. I can currently accomplish this by using version 0.28.x to build my scripts, but I would rather not be stuck on an old version forever.

Please add a flag to force the output of the build into a traditional-style .js regardless of what the input is, just like the behavior of v0.28.x.