Composer v2 compatibility

[JeppeKnockaert]

Q	A
Branch	master for features and deprecations
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Documented?	yes
Fixed tickets	#756

This pull request adds support for Composer v2. I've also added the version of composer to the test matrix, so both versions are tested.

• Wrong namespaces don't get autoloaded anymore in Composer v2, so I fixed that as well.
• Paratest has not created a version supporting composer v2 yet, this is why I included the dev requirement
• friendsofphp/php-cs-fixer has an open PR for composer v2 (Updated composer.json to support composer/semver:2.* PHP-CS-Fixer/PHP-CS-Fixer#4935), but doesn't even seem to be needed anymore in the dev requirements, so I left it out. The alternative would be to include a phar version of php-cs-fixer for Travis.
• hirak/prestissimo should not be installed anymore for composer v2 (this is now built-in), see update composer plugin api version hirak/prestissimo#218
• I don't think we need to use deactivate/uninstall (for now), because cleanup is already done by using events. Maybe things can be cleaned up / optimized after composer v1 has been dropped.

[veewee]

Thanks for the PR! It looks really nice.

The error is related to a broken openssl on appveyor, will have to take a look at why it fails before I merge it in.

[JeppeKnockaert]

Thank you @veewee. It seems every recent PR has this issue. I believe it is linked to this PHP issue, which was fixed in 7.4.6RC1 according to the comments in that issue (https://bugs.php.net/bug.php?id=79589).

[veewee]

yeah .. maybe best to hardcode it to a previous 7.4 version for now?

[veewee]

Thanks again!