Revision f444b1539523b5932ab59bab06ae25aaa230db58

feature #313 Allow building bootstrap.php.cache when app/autoload.php is absent (BPScott)

This PR was merged into the 5.0.x-dev branch.

Discussion
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Allow building bootstrap.php.cache when app/autoload.php is absent

If app/autoload.php is absent then use the autoload.php in Composer's
vendor directory.

symfony/symfony#21837 opens the door for removing `app/autoload.php` in Symfony apps. I tried removing that and updating the various reference to it in symfony-standard and my app works (:tada:), but when I run composer install/update it complains about the buildBootstrap task expecting to find `app/autoload.php`.

This fix tells the buildBootstrap task to use `vendor/autoload.php` if `app/autoload.php` is absent.

Commits
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257ee2e Allow building bootstrap.php.cache when app/autoload.php is absent