RFR: JDK-8223355: Redundant output by javadoc [v2]

[Jonathan Gibbons]

> This change addresses a bug that was observed while playing with small demo API: some files may be generated twice.
> 
> The fix is to build a single set of the files to be generated.
> 
> Note: this bug also affected the generation of the main JDK API docs.  According to the log file, before the fix we were generating 14812 files; now, we are generating only 10146 files! That's a drop of 4466 files or over 30%!

Jonathan Gibbons has updated the pull request incrementally with one additional commit since the last revision:

  Add a default-on check that "chatty" lines in the output are unique
  (primarily about generating files)
  
  Remove a redundant misleading "Generating" message: it only showed the base filename and if it had shown the full name, it would have been a duplicate

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2652/files
  - new: https://git.openjdk.java.net/jdk/pull/2652/files/5a20e8e7..186993e7

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2652&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2652&range=00-01

  Stats: 68 lines in 2 files changed: 55 ins; 9 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2652.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2652/head:pull/2652

PR: https://git.openjdk.java.net/jdk/pull/2652