Get rid of DIRECTORY_SEPARATOR

This is a first tentative of standardizing on `/` as the directory separator (in Windows too...) and render the usage of DIRECTORY_SEPARATOR unneeded.

It **seems** to work but thorough testing is needed, especially under Windows.

I'm unsure of what I did with unit tests: they should be particularly screened and tested
Reverted to its initial state in https://github.com/smz/joomla-cms/commit/ee8c39397eb147fbd5214aa0d0df988e7a5343a5