[DI] Xml definitions: support setting the injected id as node value

| Q                | A
| ---------------- | -----
| Bug report?      | no
| Feature request? | yes
| BC Break report? | no
| RFC?             | yes
| Symfony version  | 3.4

So that one could use `<argument type="service">logger</argument>`, when only `<argument type="service" id="logger" />` is supported right now. I made the mistake so many times.
