Add support for Composer 2

[villfa]

humbug/box cannot be installed with Composer 2 (see below). More details here.

$ composer --version
Composer version 2.0.0-alpha2 2020-06-24 21:36:18
$ composer install
No lock file found. Updating dependencies instead of installing from lock file. Use composer update over composer install if you do not have a lock file.
Loading composer repositories with package information
Updating dependencies
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - ocramius/package-versions[dev-master, 1.10.0, ..., 1.10.x-dev] require php ^7.4.7 -> your php version (7.2.0; overridden via config.platform, actual: 7.4.3) does not satisfy that requirement.
    - ocramius/package-versions[1.6.0, ..., 1.9.x-dev] require php ^7.4.0 -> your php version (7.2.0; overridden via config.platform, actual: 7.4.3) does not satisfy that requirement.
    - ocramius/package-versions[1.4.0, ..., 1.4.2] require composer-plugin-api ^1.0.0 -> found composer-plugin-api[2.0.0] but it does not match the constraint.
    - ocramius/package-versions[1.5.0, ..., 1.5.1] require php ^7.3.0 -> your php version (7.2.0; overridden via config.platform, actual: 7.4.3) does not satisfy that requirement.
    - ocramius/package-versions 1.99.x-dev is an alias of ocramius/package-versions dev-master and thus requires it to be installed too.
    - Root composer.json requires ocramius/package-versions ^1.4 -> satisfiable by ocramius/package-versions[1.4.0, ..., 1.99.x-dev (alias of dev-master)].

ocramius/package-versions only provides support for Composer 2 in 1.8+, which requires PHP 7.4.
If you can not upgrade PHP you can require composer/package-versions-deprecated to resolve this with PHP 7.0+.

You are using a snapshot build of Composer 2, which some of your plugins seem to be incompatible with. Make sure you update your plugins or report an issue to them to ask them to support Composer 2. To work around this you can run Composer with --ignore-platform-req=composer-plugin-api, but this may result in broken plugins and bigger problems down the line.

[villfa]

Closed by accident by using push force. I wanted to see if the tests pass without modification.

[Edit] the tests fail even without modification: https://travis-ci.org/github/humbug/box/builds/707399989

[villfa]

FYI the tests fail when executed with Composer 2 since it generates more files:

• vendor/composer/installed.php
• vendor/composer/InstalledVersions.php
• vendor/composer/platform_check.php

[theofidry]

Thank you @villfa sorry I somehow missed it. I'll come back to it next week with hopefully another update for PHP-Scoper

[villfa]

make box fails because of php-scoper. See: humbug/php-scoper#404

[theofidry]

Thank you @villfa , sorry it took so long