btw, it is also preferable (performance-wise) to use directories with trailing slash. saves us one DIRECTORY_SEPARATOR concatenation. And actually shows a small but measurable benefit. It would also be more consistent to always sstore everything with a trailing separator.

On the other hand,
- do we want people to always specify the directory with trailing slash, or would we append it? The former would be an API change, at least for PSR-0 this would not be acceptable.
- on getPrefixesPsr4() do we want to return the directories with slash or without?