Improve support for composer managed packages

* Extract logic for installing dependencies listed in a composer.json
  file into ::php::composer::install resource type.
* Add the ::php::composer::require resource type which can automate
  `composer require` and `composer remove` commands.
* Allow for future configuration of composer directories via hiera.

Change-Id: I85b7802ba3b016993586c40900969ae8aa5ec5a8
Merge "Improve support for composer managed packages"