Added suggestions for missing packages

[przemyslaw-bogusz]

Currently, when someone runs one of the most common commands, e.g. server:run, but does not have a required package installed, they will get a general 'There are no commands defined...' message.

This commit adds a more useful message, informing the user about a package that might be missing and suggesting a command, e.g. composer require symfony/web-server-bundle --dev.

[symfony-bot]

Thanks for your pull request! We love contributions.

However, this repository is what we call a "subtree split": a read-only copy of one directory of the main Symfony repository. It is used by Composer to allow developers to depend on specific Symfony components.

If you want to contribute, you should instead open a pull request on the main repository:

https://github.com/symfony/symfony

Thank you for your contribution!

PS: if you haven't already, please add tests, and beware that bug fixes should be submitted on the lowest maintained branch where they apply; only features should be submitted against the master branch.