Use PHP-provided php.ini as default

[sophiewigmore]

Context

In the restructure work (#324), the PHP Dist buildpack is responsible for configuring the default php.ini file.
The PHP Dist buildpack (as it exists in #341 for the restructure work) creates 2 php.ini files:

1. A static default php.ini file (config/default.ini)
2. A go-templated buildpack-provided php.ini (config/buildpack.ini)

Issue

This issue is to figure out how we can access the default php.ini file that ships with the PHP distribution, and use that in place of the static config/default.ini file. This is necessary because the default configuration might change slightly from version to version, so it may be problematic to use the same template for every version as we currently do.

The PHP source (from https://www.php.net/distributions/php-VERSION.tar.gz) we use to compile the buildpack-ready PHP distribution contains a php.ini-production file which could be used as the default php.ini. We should figure out we can retrieve that php.ini in the compiled PHP version, and swap that file in for the config/default.ini in the buildpack.

Acceptance

In order to close out this issue, I would expect that the default php.ini file used in the buildpack is taken from the PHP dependency from https://deps.paketo.io/php/... for all PHP versions we support in the buildpack.

[sophiewigmore]

This issue is blocked on the completion of #324