[DX] [CRUD] Added support for entity name autocompletion in the CRUD generator

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

As in the doctrine:generate:entity command, there is now support for entity name autocompletion in the form ```VendornameBundlenameBundle:MyEntity```.
Entities are filtered and fetched from the src/ folder and thus, vendor entities are excluded. Repositories are also excluded from the entity list. Autocompletion works well, even entities which are in sub-directories.
Regarding the service, it actually made testing much easier (simpler mock injection, and it needs an entity manager instance anyway, so it's much cleaner to isolate this dependency). Moreover, the service can also be used by anyone who wishes to create his own generator (which happens quite often).