Incorrect error message when --out is set to the folder containing the entry point

[Renpona]

Preflight Checklist

• [x] I have read the contribution documentation for this project.
• [x] I agree to follow the code of conduct that this project follows, as appropriate.
• [x] I have searched the issue tracker for a bug that matches the one I want to file, without success.

Issue Details

• Electron Packager Version:
  • 18.1.3
• Electron Version:
  • 29.1.0
• Operating System:
  • Windows 10
• Last Known Working Electron Packager version::
  • N/A

Expected Behavior

If the --out flag is used to set the target directory to the same directory that contains the entry point, then running electron-packager should present an error message explaining that this is not supported.

Actual Behavior

If the --out flag is used to set the target directory to the same directory that contains the entry point, then running electron-packager will result in a "The main entry point to your app was not found" error message, which does not correctly identify the problem.

To Reproduce

Create a directory structure like this:

root/
    package.json
    /dist
        code.js

In package.json, set the entry point with "main" to "dist/code.js", "./dist/code.js", or "dist\code.js".

Now run electron-packager with the command-line

electron-packager . test --platform win32 --out ./dist

The error it will return is:

The main entry point to your app was not found. Make sure "dist\code.js" exists and does not get ignored by your ignore option

This is a misleading error message for the user, however, as changing the --out target to a different directory resolves the problem.

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