The current `TermIdsResolver` can only be used to resolve the terms of a single entity, otherwise you don’t know which terms in the return value belong to which entity. We want to introduce some other service with input and output like the following:
```lang=php
[ 'Q123' => [ 0, 1, 2 ], 'Q456' => [ 7, 8 ] ]
⇒
[
'Q123' => [
'label' => [ 'en' => [ 'some label' ] ],
'alias' => [ 'en' => [ 'some alias' ], 'de' => [ 'noch ein Alias' ] ],
],
'Q456' => [
'label' => [ 'en' => [ 'a label' ], 'de' => [ 'eine Beschriftung' ] ],
],
]
```
It doesn’t need to know what the keys mean, it just preserves the association between the keys and the terms / term IDs – while ideally still retrieving all the terms, for all the keys, in bulk. (Though a simple implementation can of course iterate over the top-level array and call `TermIdsResolver::resolveTermIds` separately each time.)
Could be part of `TermIdsResolver` (either as an additional method or even replacing the current `resolveTermIds`) or a separate interface; in either case, though, the database implementation should be in `DatabaseTermIdsResolver`, not some other class.