Use normalized path

[ernilambar]

Fixes https://github.com/WordPress/plugin-check/issues/434

• Use normalized path when fetching list files and folders

[ernilambar]

@felixarntz May be using this approach will avoid fixing Windows issues randomly.

[swissspidy]

This seems to theow some errors, the function might not be available in some contexts. See https://github.com/swissspidy/wp-plugin-check-action/issues/68