RFR: JDK-8253735: Cleanup SearchIndexItem API [v2]

[Jonathan Gibbons]

> This pull-request is for a substantial refactoring and cleanup of the code
> to build the static index pages and the files for the interactive search
> 
> There is no significant change in functionality, and all tests continue to
> pass without change.
> 
> ## Improvements
> 
> * `SearchIndexItem` is merged into `IndexItem`
> * `SearchIndexItems` (the collection of `SearchIndexItem`s indexed by
>     `Category`) is merged into `IndexBuilder`, which now maintains
>     both maps: index items grouped by first character, and grouped by category
> * In `Category`, the members `INDEX` and `SYSTEM_PROPERTY` are merged into
>     single new entry `TAGS`, such that the members of `Category` now directly
>     correspond with the JavaScript files generated for interactive search
> * `IndexItem` now provides access to the `DocTree` node for those items
>     that previously were `SearchIndexItem`. This can be used to differentiate
>     between items for `{@index...}` and `{@systemProperty}`.
>     This specific change was a primary motivation for all this work, in order
>     to facilitate supporting additional new tags that can contribute items
>     for the index.
> * All index items (i.e. including those that previously were `SearchIndexItem`)
>     are now created by static factory methods instead of directly calling
>     constructors. This allows many values to be precomputed and stored in
>     final fields or made available by overriding accessor methods in
>     anonymous subtypes.
> * The comparators for index items have been cleaned up. Previously, one
>     of the comparators was "unstable", meaning that it could fall back on
>     the value of mutable fields, and the `toString` output (which was used
>     to generate the content for the JavaScript files.)
> * Work to generate the JavaScript files has been moved out of
>     `AbstractIndexBuilder` and its subtypes into a new class, `HtmlIndexBuilder`,
>     that subclasses the main `IndexBuilder`.
>     To facilitate that, some methods were moved from `HtmlDocletWriter` to
>     `Links`. This is not ideal, because it introduces a dependence on `Utils`
>     in `Links` that was not there before, but the choice was pragmatic and the
>     least bad of the alternatives.  Long term, we might want to move most
>     of the `formats/html/markup` package into a new more standalone package that
>     does not rely on other javadoc internals, and at that time, the factory
>     objects like `Links`, `TableHead` and `Table` would just move up to the
>     `formats/html` package.
> 
> ## The Changes
> 
> The work is done in a series of steps/commits, each with a specific focus
> of the work involved. It may be instructive to review the changes in each
> commit, to follow the overall evolution of the work. From the Git log,
> the changes are as follows (oldest first)
> 
> * Move `SearchIndexItem`, `SearchIndexItems` to `toolkit.utils`
> * Cleanup `SearchIndexItem`
> * fix bug
> * Simplify `SearchIndexItems`
> * simplify statement
> * Merge `SearchIndexItems` into `IndexBuilder`
> * Cleanup `IndexItem` prior to merge with `SearchIndexItem`
> * Cleanup `SearchIndexItem` prior to merge with `IndexItem`
> * Merge `SearchIndexItem` into `IndexItem`
>     (without changing how items are added to the index collections)
> * simplify adding index items to the index builder
> * move comparators for index items into IndexBuilder
> * improve init for some index items
>     improve comments in IndexItem
> * Bug fix: obsolete call to add an item to the index
>     Improve comparators used to build index
> * Move `getAnchor` methods from `HtmlDocletWriter` to `Links`.
>     This is slightly undesirable because it means that `Links` requires access to `Utils`, but it is less bad than the
>     alternatives.
> * Move code to complete initialization of index items and to write JavaScript index files from `AbstractIndexWriter` and
>   subtypes to new subtype of `IndexBuilder`: `HtmlIndexBuilder`
> 
> At each stage, the repo should build and all javadoc tests should pass (and there is no reason to believe that any
> other tests may fail.)
> ## Future work
> 
> Instead of maintaining collections of `SortedSet`s in `IndexBuilder`, it might
> be more effective to use `List` instead, and just sort the list as needed.
> 
> There is little need to eagerly build both maps in `IndexBuilder`. As long as
> there is at least one collection, such as the `itemsByFirstCharacter`, we could
> defer generating `itemsByCategory` until we need to write out the JavaScript
> files.  There is one place in the code, in `SystemPropertyWriter`, where we
> look at `itemsByCategory` to determine whether there were any
> `{@systemProperty...}` tags, but at the time we need that information, it
> would not be significantly more expensive to scan `indexByFirstCharacter`,
> because the items for elements need not have been added at this time.

Jonathan Gibbons has updated the pull request incrementally with two additional commits since the last revision:

 - Address review comments
 - Fix minor typo

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/499/files
  - new: https://git.openjdk.java.net/jdk/pull/499/files/5343e741..670afb13

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=499&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=499&range=00-01

  Stats: 39 lines in 8 files changed: 21 ins; 5 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/499.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/499/head:pull/499

PR: https://git.openjdk.java.net/jdk/pull/499