Allow symfony/finder 4

[fancyweb]

The symfony/finder component is not a core Drupal dependency. So it not technically locked to ^3.4.

I have reviewed the usage that is made of this component in drush. The only thing that is used is the Symfony\Component\Finder\Finder class.

I have checked every method of this class that is used by drush. Then I have read the CHANGELOG of the component and did not see any obvious impact for drush. Then I made a diff between the 3.4 branch and the master branch of this class file and manually checked changes that could break drush usage of this component. I did not find any.

It would be great to accept v4 of this component, so projects that require drush and this component directly can use the latest version.

[greg-1-anderson]

Tests are green so I'm +1 on this.

@weitzman: The greater question is should we ensure that we actually test with multiple versions of the Symfony components? As it is, this change means that we'll be testing with ^4 all the time. This is probably fine; we could consider adding a lowest test configuration to the functional tests. That wouldn't be hard, but I don't know if it would be worth the extra time. Maybe convert the php 5.6 test to a lowest test?

[deviantintegral]

Maybe convert the php 5.6 test to a lowest test?

👍 to this.

This comes up if you run core's command to update PHPUnit for testing on PHP 7, and then try to install drush afterwards. You have to force a downgrade with composer require symfony/finder:^3.4 and then install drush, which works but isn't intuitive.