Add automatic verification for export dependencies list being in sync

Spin of #44 (and https://github.com/elastic/open-source/issues/135) for the following parts:
> ## For products not using release manager
>
> - [ ] Each build creates a file describing all dependencies (please add an example link to this issue)
>
> ## Optional but encouraged
>
> - [ ] The product build does not permit adding a dependency without automatic export in the list

# Implementation

Since the distributed (AKA production) part of APM PHP agent currently doesn't have any 3rd party dependencies the simplest approach to implement this feature would be to add automatic verification that no dependencies added to `"require"` section of [composer.json](https://github.com/elastic/apm-agent-php/blob/master/composer.json)