Make it possible to move DBTNG into a component by making the dependency to Drupal optional

Problem/Motivation

I'm told that DBTNG will eventually be split out into a component (yay!), which is good, but unfortunately it's not quite ready for that. There are still a few hardwired dependencies on various pieces of Drupal here and there. These should be removed so that DBTNG can take its rightful place in component-space.

Proposed Resolution

All references to Drupal core should be removed from DBTNG, and any Drupal-specific stuff should be implemented in Drupal-specific extensions of DBTNG.

Remaining Tasks

• Discuss
• Patch
• Review
• Commit

Attached is an initial patch of the base implementation of Select which checks for \Drupal before calling \Drupal::moduleHandler() and trying to alter the query, since altering queries is a Drupal-specific thing.