[DX] Removed strict alias name check

Symfony allows to change the DI alias by overriding `Extension#getAlias()`, but it does throw an exception when it is anything else than the default. That doesn't sound nice and it makes it harder to change the alias. This can result in problems when the bundle is called WouterJEloquentBundle for instance (which has a default alias of `wouter_j_eloquent_bundle`, where I want it to be `wouterj_eloquent_bundle`).

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -
AFAIK, that would be the first usage in a Symfony exception message.