[Console] clean handling of :: passed to find()

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12104
| License       | MIT
| Doc PR        |

Commands cannot have a double colon in their name (for example, a command can't be named `foo::bar`). However, one might try to retrieve a `foo::bar` command from the application like this:

```php
$command = $application->find('foo::bar');
```

The `findAlternatives()` method of the `Application` class fails to handle these strings when there are commands registered with a name consisting of at least three parts (e.g. a command is named `foo:bar:baz`). In this case, an empty string is passed to `strpos()` causing PHP to raise a warning.

In a "real" Symfony application, calling `php ./app/console cache::clear` now results in the following error message:

```
  [InvalidArgumentException]                                
  There are no commands defined in the "cache:" namespace.  
  Did you mean this?                                        
      cache
```