join module should not depend on grouping module [LUCENE-3997]

I think TopGroups/GroupDocs should simply be in core?

Both grouping and join modules use these trivial classes, but join depends on grouping just for them.

I think its better that we try to minimize these inter-module dependencies.
Of course, another option is to combine grouping and join into one module, but
last time i brought that up nobody could agree on a name.

Anyway I think the change is pretty clean: its similar to having basic stuff like Analyzer.java in core,
so other things can work with Analyzer without depending on any specific implementing modules.

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Migrated from [LUCENE-3997](https://issues.apache.org/jira/browse/LUCENE-3997) by Robert Muir (@rmuir), updated May 09 2016
Attachments: [LUCENE-3997.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-3997/LUCENE-3997.patch) (versions: 2)

Review the core, grouping, and join modules along with the TopGroups and GroupDocs classes; the attached LUCENE-3997.patch provides prior context. Start by tracing the module dependency and usages of these classes. Done means join no longer depends on grouping while both modules continue to use the shared classes correctly.

Written by the indexing model from the issue text.