Command discovery does not working in directories that contain files other than javascript

[thesayyn]

Describe the bug
program.discover does not work with typescript compilation outputs.

To Reproduce
Steps to reproduce the behavior:
1- Create a typescript project
2- Create a program
3- Create a directory called commands
4- Put a command into the commands directory
5- Enable command discovery via the discover option.
6- Compile the project

// Javascript or Typescript code to reproduce bug
import {Program} from "@caporal/core";
import * as path from "path";

new Program()
  .discover(path.join(__dirname, "commands"))
  .run();

Expected behavior
Command discovery should not try to require non-javascript files such as typescript declaration files.

Actual behavior
Command discovery is trying to require types such as typescript declarations.

Environment informations (please complete the following information):

• OS: Mac
• OS version: Catalina
• Shell: zsh
• Caporal version: 2.0.2

Additional context
Add any other context about the problem here.

[thesayyn]

@mattallty 👋, Any thoughts on this?

[electrotype]

Related issue here. I have a Typescript project where the .js files are generated beside their associated .ts files.

The commands work, but the global help show each command two times instead of one.

[gregorskii]

New to this lib, but looking at the source in scan I think I see the issue:

https://github.com/mattallty/Caporal.js/blob/master/src/command/scan.ts#L19

This needs to be scoped to include only files that contain commands, thinking more on this it can't just be JS, is there some convention that could be used? {name}.command.js?