[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.
I'm mixed on whether this should be a service or not. Obviously, this allows this to be overridden more easily by the user, but I don't really think that's a realistic use-case. On the negative side, this makes the container just a little bit bigger, without much/any benefit.

My vote would be to *not* have this as a service, and to instantiate it directly in `GenerateDoctrineCommand`. But perhaps others will disagree!