Update to support Composer 2.0

[robbieaverill]

We are using Composer's API to process a project's dependency list. Composer 2.0 is now available, and part of this is:

There is a new class, Composer\InstalledVersions, which is autoloaded in every project and is available at runtime. It allows you to check which packages/versions are present at runtime of your own project.

If nothing else this will simplify this module - in theory we can use this class to access the information we're currently building semi-manually with the Composer API.

[GuySartorelli]

If your code relies on any of these runtime features, you should require "composer-runtime-api": "^2.0" in your composer.json. This is a virtual package which is provided by Composer and will make sure people have to use Composer 2.x to install your package.

Considering Silverstripe Cloud and Public Sector still (afaik) use composer 1, is this a change that would be okay to make?

[GuySartorelli]

If nothing else this will simplify this module - in theory we can use this class to access the information we're currently building semi-manually with the Composer API.

It turns out all this gives you is the currently installed version (which we could just fetch from composer.lock anyway) - it doesn't provide the contraints enforced by dependent packages.

[GuySartorelli]

Will be resolved by #62

[GuySartorelli]

The new major version uses composer 2. This ticket should be closed.