[tests] use composer to require maker bundle in tests

[jrushlow]

Instead of copying MakerBundle to the flex skeleton and fiddling with autoloader to force things to work. Let's:

• add MakerBundle as a "path" repository to the skeletons composer.json
• composer require --dev symfony/maker-bundle
• Symlink Maker's src/ to the skeleton's vendor/symfony/maker-bundle/src directory so we can test changes locally without having to create a commit.

Caveats:

• In Windows, git clone and symlinks are problematic. I experienced problems creating a symlink in the skeleton then attempting to clone the skeleton for individual maker tests. Workaround: composer require --dev symfony/maker-bundle + create the symlink each time we generate a "project" from the flex_skeleton

• Windows AppVeyor test times have a ~4 min increase across the whole suite. This will likely be reduced when we move Windows CI to GitHub Actions.

Side note: We're doing this so we can php-cs-fixer in each maker run w/o having to screw with the autoloader for symfony/process