Upgrade phpunit/php-code-coverage to 9.0

[nreynis]

This package dependencies prevent upgrading to phpunit 9.3.
Being stuck with phpunit 9.2 is an issue because it also lock us away from PHP8.

[matthiasnoback]

Makes sense, can you make a PR?

[nreynis]

I tried to give it a shot, but there's major BC in php-code-coverage.
I'm not familiar with previous version, so I'll have to understand what each method was doing and how it maps to the new API. If I have some time I'll try to dig further, but I can't promise anything.

[nreynis]

Done!

[matthiasnoback]

Fixed by #16.

[matthiasnoback]

Thanks a lot Nicolas! I've added one other commit because I noticed that PHPUnit now has a FilterMapper that does the work of processing the files and directories etc. inside the config. So that saves us a few lines here.

[matthiasnoback]

By the way, I've released this as v1.4.0.