RFR: 8346659: SnippetTaglet should report an error if provided ambiguous links [v4]

[Nizar Benalla]

> Some javadoc snippets can match multiple links to the same content, leading to different results in different javadoc runs.
> This patch proposes emitting an error when such cases are encountered.
> 
> There is a very trivial, unrelated change in `TestGlobalHtml.java` because I noticed some whitespace wasn't right.

Nizar Benalla has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:

 - Merge remote-tracking branch 'origin/master' into snippet-non-rep
 - Merge remote-tracking branch 'upstream/master' into snippet-non-rep
 - adjusting error message
 - Merge remote-tracking branch 'upstream/master' into snippet-non-rep
 - emit error when encountering ambigious link

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/23328/files
  - new: https://git.openjdk.org/jdk/pull/23328/files/5ee95f7e..e06fbf4d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23328&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23328&range=02-03

  Stats: 15265 lines in 628 files changed: 10931 ins; 2046 del; 2288 mod
  Patch: https://git.openjdk.org/jdk/pull/23328.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23328/head:pull/23328

PR: https://git.openjdk.org/jdk/pull/23328