Fix get-plugin-dir command for plugins

[mukeshpanchal27]

Summary

Follow-up #1000

In the changes made in 15b1cc0, we simplified the commands get-plugin-dir and get-plugin-version. However, when we run node ./bin/plugin/cli.js get-plugin-version --slug=auto-sizes for the manual workflow, it shows an error: Error: The "auto-sizes" module/plugin slug is missing in the file.

This PR resolves that issue.

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Co-authored-by: mukeshpanchal27 <mukesh27@git.wordpress.org>
Co-authored-by: felixarntz <flixos90@git.wordpress.org>
Co-authored-by: thelovekesh <thelovekesh@git.wordpress.org>

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[felixarntz]

@mukeshpanchal27 Good catch!