Allow Symfony 3 components for site-local installs of Drush 8.x

[greg-1-anderson]

While keeping global installs at Symfony 2.

If we do not allow Symfony 3 components, then site-local installs of Drush 8.x will not work with Drupal 8.4.x. However, if we simply add Symfony 3 to the require section, then we will break global installs of Drush for Drupal 8.3.x. To prevent this, we pin Symfony components to Symfony 2 in require-dev.

c.f. #2799

[greg-1-anderson]

Drush 8.x will not support site-local installation for Drupal 8.4.x or later. Use Drush 9 for that.

[greg-1-anderson]

Let's revisit this for 8.1.15

[greg-1-anderson]

Here are the installation permutations with this PR:

• composer require of Drush8 outside a Drupal site gets Symfony 2. Global Drush 8 w/ Symfony 2 works with Drupal 8.3 and Drupal 8.4.
• composer require of Drush8 inside a Drupal 8.4 site gets Symfony 3
• composer require of Drush8 inside a Drupal 8.3 site gets Symfony 2
• The phar gets Symfony 2, just like the first example

The composer.lock file has Symfony 2.

[grasmash]

composer require of Drush8 inside a Drupal 8.4 site gets Symfony 3

Much needed!

[xjm]

Added a followup for this in core: https://www.drupal.org/node/2916204