Add support for ext-parallel

[jwage]

What do you think about adding support for ext-parallel?

https://github.com/krakjoe/parallel

This extension also requires zts.

Would you accept a pull request for it?

[dzuelke]

Yup, I'd love to.

ZTS means we need to support an entirely separate series of ZTS PHP and extension builds. This is not trivial work, because it's yet another "dimension" for the build and install infrastructure to support, but unlike the stack, it can't be known ahead of time, so you can't have separate package repositories, but instead need ZTS and NTS packages in the same package repository, with suitable require and conflict entries, different effective package filenames, etc.

An upcoming project for this year (and soon) is to overhaul the way packages are built for both the Classic buildpack and the PHP CNB, since we want to support different architectures as well, and also have a known state of available packages attached to a particular release, instead of the current "all that's in the bucket" approach.

But this work will also make it much easier for us to support many more extensions, since most of the "management" around updates should be automated, and the generation and syncing of repositories should handle large amounts of packages much better.

Tracking notes here: #695