RFR: 8335122: Reorganize internal low-level support for HTML in jdk.javadoc [v7]

[Jonathan Gibbons]

> Please review a change to reorganize the internal low-level support for HTML in the jdk.javadoc module.
> 
> Hitherto, there are two separate sets of classes for low-level support for HTML in the `jdk.javadoc` module: one, in doclint, focused on reading and checking classes, the other, in the standard doclet, focused on generating HTML. This PR merges those two sets, into a new package `jdk.javadoc.internal.html` that is now used by both `doclint` and the standard doclet.
> 
> There was a naming "anti-clash" -- `HtmlTag` in `doclint` vs `TagName` in the standard doclet. The resolution is to use `HtmlTag`, since the merged class is more than just the tag name.
> 
> A few minor bugs were found and fixed.   Other minor cleanup was done, but otherwise, there should be no big surprises here. But, one small item of note: `enum HtmlStyle` was split into `interface HtmlStyle` and `enum HtmlStyles implements HtmlStyle` to avoid having a doclet-specific enum class in the new `internal.html` package.  The naming follows `HtmlId` and `HtmlIds`.
> 
> There is no attempt at this time to simplify `HtmlTag` and `HtmlAttr` to remove support for older versions of HTML.

Jonathan Gibbons has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 12 commits:

 - Merge with upstream master;
   add reference in HtmlTag to a related RFE
 - Update/improve merge of HtmlTag and TagName
 - Fix missing legal headers
 - Cleanup use of HtmlStyle and HtmlStyles
 - Fix merge issues
 - Merge with upstream master
 - fix whitespace
 - reorder imports
 - Cleanup pass
 - Merge doclint into html: part 2: TagName/HtmlTag
 - ... and 2 more: https://git.openjdk.org/jdk/compare/e4c7850c...2b6363ee

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Changes: https://git.openjdk.org/jdk/pull/19916/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19916&range=06
  Stats: 3024 lines in 108 files changed: 1164 ins; 1221 del; 639 mod
  Patch: https://git.openjdk.org/jdk/pull/19916.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19916/head:pull/19916

PR: https://git.openjdk.org/jdk/pull/19916