Rename some classes and methods in the search package so their names describe their roles [LUCENE-3173]

In an IRC discussion with Robert Muir, it has come up that the names of certain classes and methods in the search package do not describe the role of the class or method in question. Therefore, we propose the following name changes:
- Scorer to Matcher
- Similarity.computeWeight() to Similarity.computeStats()
- Similarity.DocScorer to Similarity.Scorer\*

\*: Under consideration.

The reason for the name computeStats() is that this method will be used by the new Similarity subclasses to compute all the statistics they need, not just idf.

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Migrated from [LUCENE-3173](https://issues.apache.org/jira/browse/LUCENE-3173) by David Mark Nemeskey
Parent: #4033
Attachments: [irc.log](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-3173/irc.log)

Start in the search package by locating Scorer, Similarity.computeWeight(), and Similarity.DocScorer, then read the parent issue #4033 and the attached IRC discussion for the naming context. Done means the agreed names are applied consistently across their references and the relevant search tests pass; confirm whether the DocScorer rename is still under consideration.

Written by the indexing model from the issue text.