AssetMapper: upgrade packages when needed

[weaverryan]

Hi!

The scenario:

• User installs symfony/ux-autocomplete. Flex adds tom-select at version 2.2.3 to importmap.php
• 3 months later, user upgrades symfony/ux-autocomplete. The new version now requires tom-select at ^2.5.

If we do nothing, the user's tom-select is out of date and the user won't even know about it. The ^2.5 constraint is defined in the symfony/ux-autocomplete package.json file, but nothing enforces that or notifies the user.

With this small change, during a composer update/require, we look at the dependencies of all UX packages and compare them against the version in importmap.php. If they do not match, the package is upgraded to a version that matches.

We're not creating a fully-fledge package management by any means, but we don't need to. With a few notifications and things like this, we can keep the user's dependencies in sync with each other.

Unrelated: don't forget about that really cool other PR #978 ;)

Cheers!

[weaverryan]

fabbot failures are incorrect - they're trying to add PHP 8 only functions, while 1.x works on 7.1.

[fabpot]

Thank you @weaverryan.