Use DIRECTORY_SEPARATOR instead of slash in filepaths

[mnelson4]

Description of the Change

Use the PHP constant [DIRECTORY_SEPARATOR](https://www.php.net/manual/en/dir.constants.php) (which is / on *nix, and \ on windows) instead of / in filepaths.
Without this, the plugin doesn't work on Windows machines: EDW thinks the modules aren't defined and so doesn't try to activate them, so the "Print" admin page doesn't appear. See https://i.imgur.com/9n1CqO0.png

Alternate Designs

You could alternatively do a string replace to standardize which slashes are used. This seemed simpler.

Benefits

EDW can now work on Windows machines, including localhost installs.

Possible Drawbacks

Verification Process

I tested this on Windows (see https://i.imgur.com/ANd5ut7.png) and on a Linux machine.
I'm able to create a print edition zip.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation. --> I don't think it does
• I have updated the documentation accordingly. --> N/A
• I have added tests to cover my change. -->I have not added tests, I don't think this is necessary
• All new and existing tests passed. --> you talking crazy talk; what tests?

Applicable Issues

Should address #73

[dinhtungdu]

I can't test on Windows but the code looks good to me!