First PHP CNB implementation

[dzuelke]

Still to do, coming in separate PRs:

• some trivial validation (see validate in php_project.rs)
• output filtering for composer and other invocations
• some refactoring of PlatformLayer in conjunction with corresponding changes to the Composer Installer Plugin

Pretty much all TODO or FIXME comments are either in reference to the above items, or longer-term notes.

Related: https://github.com/heroku/heroku-buildpack-php/pull/new/cnb-installer