RFR: JDK-8223355: Redundant output by javadoc

[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 446 files or over 30%!

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Commit messages:
 - JDK-8223355: Redundant output by javadoc

Changes: https://git.openjdk.java.net/jdk/pull/2652/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2652&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8223355
  Stats: 76 lines in 2 files changed: 73 ins; 0 del; 3 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