[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 thought you said you didn't create an interface? ;)

I agree with your comment here: https://github.com/sensiolabs/SensioGeneratorBundle/pull/216#issuecomment-57919508 - the interface is overkill, and we're not really using it (i.e. there is no spot where we accept this interface as an argument). Let's remove it.