Explainable ConstantScoreQuery [LUCENE-9430]

Developer cannot check the result of detailed explain with ConstantScoreQuery.

If the developer want to check it, then must be change the query.

The result is the same as ConstantScoreQuery,

but if you can check the detailed explain, it will help in development and debug.

So, I make the Pull Request for this :)

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Migrated from [LUCENE-9430](https://issues.apache.org/jira/browse/LUCENE-9430) by Jehyun Lee, updated Jul 16 2020
Pull requests: https://github.com/apache/lucene-solr/pull/1674

Start at the ConstantScoreQuery entry point and inspect how its detailed explanation is currently produced. Review the existing pull request linked in the issue and related explanation tests, then verify that developers can inspect detailed explain output without changing the query.

Written by the indexing model from the issue text.