RFR: 8349369: test/docs/jdk/javadoc/doccheck/checks/jdkCheckLinks.java did not report on missing man page files [v2]

[Nizar Benalla]

> Please review this small update to make the test more robust, the test did not always check links to other files if they did not contain a fragment or a `#`symbol.
> 
> I'd like to add that the original [doccheck](https://github.com/openjdk/doccheck) also didn't pick up on these missing files when I ran it. 
> 
> TIA

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:

 - improve based on feedback
 - Merge branch 'master' into update-link-checker
 - Merge branch 'master' into update-link-checker
 - Merge branch 'master' into update-link-checker
 - update link checker to verify that files exist

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/23441/files
  - new: https://git.openjdk.org/jdk/pull/23441/files/b0fcb616..95eb3cec

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23441&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23441&range=00-01

  Stats: 658029 lines in 9092 files changed: 275680 ins; 327663 del; 54686 mod
  Patch: https://git.openjdk.org/jdk/pull/23441.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23441/head:pull/23441

PR: https://git.openjdk.org/jdk/pull/23441