Use common directory structure

[szepeviktor]

Type: refactoring
Breaking change: no

1. Use common directory structure as seen in thousands of PHP packages
2. Use PSR-4 autoloader (namespace\ = src/)

🤩

What do you think about it?
Is it worth to finish?

[AJenbo]

I like the cleaned up structure

[kylekatarnls]

This will conflict with all our open pull-requests :(

[szepeviktor]

@kylekatarnls This is really an inspiration focusing on three root directories: src, tests, website.
Please manage it the way you want.

[AJenbo]

I belive git is actually able to resolve most of that automatically... though maybe not github

[ravage84]

I think we should look into this once the current dust of changes has settled on the next major version.

Also, I would go one step beyond the "common directory structure as seen in thousands of PHP packages".
We can try to adhere to the directory structure standard of PHP PDS.