Add support for Composer ^2.0

[jeromegamez]

Closes #67, supersedes #61

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Greetings 👋,

I love your package and use it ev.ery.where, thank you so much for it!

Since I'd like to check out the upcoming Composer 2.0 release, this PR aims to add support for it. I tried to be as unobtrusive with my changes as possible, but also thought that this might be an opportunity to add some other updates.

Notable changes

Composer ^2.0

The plugin was pretty much 2.0-ready already 🥳, only the tests needed some adaptions. The Plugin now has empty deactivate() and uninstall() methods, and I wasn't able to figure out what to fill them with 🤷‍♂️.

PHP CS Fixer ^2.16

I had to remove the dev-dependency because PHP CS Fixer is currently limited to composer/semver: ^1.4.

Once PHP-CS-Fixer/PHP-CS-Fixer#4935 is addressed, it could be added back, but in the meantime, this is resolved on Travis CI with an ad-hoc download of the PHAR, and locally e.g. with a composer global require friendsofphp/php-cs-fixer

PHPUnit ^7.5

I added : void to the setUp() and tearDown() methods to ease the migration path to PHPUnit 8/9.

There was a mixture of $this->assert* and static::assert*, so I unified all calls to use $this (just my personal preference 😅)

@expectException* annotations have been deprecated, so I replaced them with $this->expectException*

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I hope that the changes meet with your approval, and I'm looking forward to your review! 🌺

[pyrech]

Thank you so much for taking some time to update this library (PR is really well described, thanks!!) 😍

Concerning $this->assert VS self::assert I would have preferred the latest version as assert methods are in fact static but a not a big deal. Most PHP devs use the former and I can live with it 😄

Let's merge this PR. I guess I can release that as a patch version (aka 1.7.1) as there is no BC break, nor new feature.

Thanks again!

[jeromegamez]

Nice!! Thank you for reviewing and merging so quickly! 🥰