minor #9523 [Setup] Add some missing items and hints about migrating from standard edition to Flex (phansys)

This PR was submitted for the master branch but it was merged into the 4.0 branch instead (closes #9523).

Discussion
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[Setup] Add some missing items and hints about migrating from standard edition to Flex

|Q            |A     |
|---          |---   |
|Branch       |master|
|Bug fix?     |no    |
|New feature? |no    |
|BC breaks?   |no    |
|Deprecations?|no    |
|Tests pass?  |yes   |
|Fixed tickets|n/a   |
|License      |MIT   |
|Doc PR       |n/a   |

Commits
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0b58449 Add some missing items and hints about migrating from standard edition to Flex

Author: javiereguiluz

setup/flex.rst

@@ -228,6 +228,12 @@ manual steps:
    :doc:`autowiring feature </service_container/3.3-di-changes>` you can remove
    most of the service configuration.
 
+.. note::
+
+    Make sure your previous configuration files don't have ``imports`` declarations
+    pointing to same resources already loaded by ``Kernel::configureContainer()``
+    or ``Kernel::configureRoutes()`` methods.
+
 #. Move the rest of the ``app/`` contents as follows (and after that, remove the
    ``app/`` directory):
 
@@ -236,8 +242,9 @@ manual steps:
    * ``app/Resources/<BundleName>/views/`` -> ``templates/bundles/<BundleName>/``
    * rest of ``app/Resources/`` files -> ``src/Resources/``
 
-#. Move the original PHP source code from ``src/AppBundle/*`` to ``src/``. In
-   addition to moving the files, update the ``autoload`` and ``autoload-dev``
+#. Move the original PHP source code from ``src/AppBundle/*``, except bundle specific
+   files (like ``AppBundle``, ``DependencyInjection\``), to ``src/``. Remove ``src/AppBundle/``.
+   In addition to moving the files, update the ``autoload`` and ``autoload-dev``
    values of the ``composer.json`` file as `shown in this example`_ to use
    ``App\`` and ``App\Tests\`` as the application namespaces (advanced IDEs can
    do this automatically).
@@ -261,6 +268,13 @@ manual steps:
 #. Update the ``bin/console`` script `copying Symfony's bin/console source`_
    and changing anything according to your original console script.
 
+#. Remove ``bin/symfony_requirements`` script. If you need a replacement for the
+   task performed by this script, use `Symfony Requirements Checker`_:
+
+   .. code-block:: terminal
+
+       $ composer require symfony/requirements-checker
+
 .. _`Symfony Flex`: https://github.com/symfony/flex
 .. _`Symfony Installer`: https://github.com/symfony/symfony-installer
 .. _`Symfony Standard Edition`: https://github.com/symfony/symfony-standard
@@ -272,3 +286,4 @@ manual steps:
 .. _`shown in this example of the skeleton-project`: https://github.com/symfony/skeleton/blob/8e33fe617629f283a12bbe0a6578bd6e6af417af/composer.json#L44-L46
 .. _`copying Symfony's index.php source`: https://github.com/symfony/recipes/blob/master/symfony/framework-bundle/3.3/public/index.php
 .. _`copying Symfony's bin/console source`: https://github.com/symfony/recipes/blob/master/symfony/console/3.3/bin/console
+.. _`Symfony Requirements Checker`: https://github.com/symfony/requirements-checker