In `org.eclipse.help.internal.search.SearchIndex`, refactor the
`openSearcher()` method to return an `AutoCloseable` object so that it
can be used in a try-with-resources statement that implicitly registers
and unregisters the search.

Minor performance improvement: Use double-checked locking idiom for lazy
initialization of the `IndexSearcher`.

Grant access to the `searches` and `analyzerDescriptor` fields via
`SearcherInfo#getIndexSearcher()` and
`SearcherInfo#getAnalyzerDescriptor()`. These two getter methods are not
(yet) used internally. Their only purpose is to be able to implement a
hybrid search that combines a semantic search with a sparse keyword
search. And the sparse keyword search uses the existing `SearchIndex`,
which requires access to these fields.