RFR: 8373909: JSpec and ToolGuide taglets use incorrect relative path [v2]

[Dan Smith]

> Fixing the JSpec and ToolGuide taglets so that they produce correct links, no matter what file the tag appears in.
> 
> This is a workaround solution, sharing the necessary info via a ThreadLocal public static field in an internal API. See [JDK-8373922](https://bugs.openjdk.org/browse/JDK-8373922) for an API enhancement request that would make this possible without relying on internal API.

Dan Smith has updated the pull request incrementally with one additional commit since the last revision:

  Factor out reflection code to run just once

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/28878/files
  - new: https://git.openjdk.org/jdk/pull/28878/files/1ad28fa3..180eea30

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28878&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28878&range=00-01

  Stats: 28 lines in 2 files changed: 16 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/28878.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28878/head:pull/28878

PR: https://git.openjdk.org/jdk/pull/28878