[Translator][FrameworkBundle] Allow remote message sources

Currently you have to create dummy empty files in yours Resources folder
when you use customized Translation Loader Interface, because loader from
Framework Bundle omits all loaders that don't have file.

This PR attempts to fix this behaviour without BC breaks;

Example usage:
You have messages stored in MongoDB, collections are yours message domains groups.
Via DI you inject Mongo connection to loader, and then through yours implementation discover avaliable domains.

| Q             | A
| ------------- | ---
| Bug fix?      | yes - by feature addition
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #6930
| License       | MIT
| Doc PR        | -

### TODO if feature gets positive feedback

- [ ] submit changes to the documentation
- [ ] write unit tests