Move to Symfony Flex

The structure changed completely.
Bundles are gone. Everything is flatten in the folder `src`.
I separated import & api controllers to avoid _pollution_ in the main controller folder.

config/packages/sensio_framework_extra.yaml

@@ -0,0 +1,4 @@
+# see https://github.com/symfony/symfony-standard/pull/1133
+sensio_framework_extra:
+   router:
+        annotations: false