Simplify and validate features relying on _source #11172
Description
Today, we have some features that depend on _source being complete. We now have the ability to know whether _source exists and has all original fields (no includes/excludes) with #11171.
We now need to add checks to features that rely on source and simplify them. For example, highlighting currently has code to try and highlight based on stored fields if source does not exist. We should not do this. As discussed in #8142, disabling source is an advanced feature, and comes with a cost. That cost is some features don't work, and we should simplify highlighting to reflect that.