Make the Project Platform/Host Agnostic

### Description

When we moved a client site from Pantheon to VIP, we had a repo set up for a few weeks that could deploy to both hosts. If it deployed to Pantheon, there was a minimal `mu-plugins` directory that would load everything in `client-mu-plugins` and the plugin loader was set up to conditionally load some of the plugins that VIP includes in `mu-plugins`. If it deployed to VIP, that `mu-plugins` directory was ignored, and things worked as expected.
We could also provide some wrapper functions for things that the hosts handle differently such as caching related functions.

### Use Case

With some of our clients bouncing between hosts or making last minute decisions on hosting due to budget and bidding wars, it made me wonder if we could somehow build a setup that would work on both VIP and Pantheon, and we could just make it our stock setup. This would make things more consistent for developers across projects as well.