XD-1585: Completions after a named channel

Adds completions for cases like
```
queue:foo > <TAB>
queue:foo > splu<TAB>
```

with one caveat: processors will not be part of proposals for the second case (XD-1830)
I don't think so because in all cases, the parser has exited the `maybeEatSourceChannel()` method. We could definitely use something different in the sample and the tests, this was not very smart of me :(

Can you change one to e.g. "tap:queue:bar" on merge?