DX-5185: Decouple BLT settings.php

[fiasco]

Is your feature request related to a problem? Please describe.
By design BLT no longer ships with Acquia CMS or the acquia/drupal-recommended-project. However it contains critical settings.php configuration that makes it critical for Drupal projects hosted on Acquia still.

Describe the solution you'd like
By decoupling blt's php settings.php auto-configuration capability it will allow non-BLT projects to leverage these settings capabilities without having to add the rest of BLT and its dependency tree to the composer project.

Additional context
Propose the project is called acquia/drupal-settings and contains minimal to no dependencies but becomes a dependency of BLT for backwards compatibility.

[bkosborne]

Curious why Acquia CMS doesn't depend on BLT?

[mikemadison13]

we are actively working on decoupling these, btw.

and @bkosborne not all Acquia customers use BLT and we didn't want to force it on them.