Restructure buildpack: PHP Start without FPM

[sophiewigmore]

BLOCKED on paketo-buildpacks/php-fpm#8

Context

PHP RFC 0003 is an addendum to PHP Restructure RFC 0001. The RFC outlines creating a PHP Start buildpack for managing the task of running start commands for HTTPD, Nginx, and FPM.

Issue

This is to add the new PHP start buildpack base implementation with support for the Nginx and HTTPD as standalone servers, without FPM in the same container. Based on the outcome from paketo-buildpacks/php-fpm#8, this buildpack will need to implement a solution for discovering that FPM should run separately, and run HTTPD or Nginx alone in a container.