DT-747: Remove dependency on webflo/drupal-core-require-dev

[danepowell]

We added webflo/drupal-core-require-dev to blt-require-dev (and by extension, the require-dev section of every BLT project's composer.json) in #3065

This has caused a fairly regular spate of issues. For one, it seems to confuse people trying to update Drupal core because they have to whitelist webflo/drupal-core-require-dev along with core if they aren't doing a general composer update.

More recently, it contributed to #3585

More generally, it just seems like an unnecessary set of dependencies for most users, since it's only necessary if you are running Drupal core test suites.

Now that we have a dedicated tests:drupal:run command that the above users running core tests should be using, I think we should remove drupal-core-require-dev as a general dependency and instead dynamically check for and help add it only as part of the tests:drupal:run command.

We'll just need to ensure that any of BLT's core dependencies are replicated outside of the webflo package. And we need to consider if this can be done in a BC way.