Fix split package org.apache.lucene.index.ShuffleForcedMergePolicy

[ChrisHegarty]

To allow for the future modularization of Elasticsearch with Java Modules, there are a number preparatory tasks that need be completed. This is one part of one such task: eliminate split packages.

Since recent Lucene upgrades, ShuffleForcedMergePolicy has but a single package-private dependency on lucene, namely SegmentInfo::setDiagnostics. SetDiagnostics is being used by SFMP to set a single additional ES specific diagnostic key. It is possible to recreate the whole SegmentCommitInfo ( and the SI within, along with the additional diagnostic key ), by using just the accessible parts of the lucene API. But this clearly results in little more object allocation.

If this object allocation is unacceptable, then we can raise this issue with Lucene, with a view to improving the Lucene API to allow for such use case that ES has - to add an additional diagnostic.

relates #78166

[ChrisHegarty]

In terms of the relocated package, org.elasticsearch.index.engine was chosen since the only usage of SFMP is from code within that package.

[ChrisHegarty]

As can be seen from maxDocOrDefault in the new code - something about re-creating the SI feels a little uncomfortable!

[jpountz]

The object allocation is no concern at all here since this method is called infrequently.

However I think we should still raise it with Lucene. The javadocs of setMergeInfo mention explicitly that this may be used to set diagnostics, so it looks like a bug to me that the javadocs reference a method that is not accessible.

[ChrisHegarty]

Closing this issue. Fixed by #78605