This makes GET operations more consistent with `_search` operations which expect
`(stored_)fields` to work on stored fields and source filtering to work on the
`_source` field. This is now possible thanks to the fact that GET operations
do not read from the translog anymore (#20102) and also allows to get rid of
`FieldMapper#isGenerated`.
The `_termvectors` API (and thus more_like_this too) was relying on the fact
that GET operations would extract fields from either stored fields or the source
so the logic to do this that used to exist in `ShardGetService` has been moved
to `TermVectorsService`. It would be nice that term vectors do not rely on this,
but this does not seem to be a low hanging fruit.
3ed0da5a58