Allow loading Psalm plugins from the application source code

[janopae]

More and more PHP libraries bring their own Psalm plugins with them (like https://github.com/CuyZ/Valinor for example). The plugin either comes as a separate composer package to be composer required by the application, or is directly integrated into the main library package.

Sadly, declaring other plugins than the ones shipped with the container is currently not possible, as namespaces in the psalm.xml of the repository we want to analyse will not refer to our repository, but to the internal namespaces of the container.