RFR: 8154399, 8159096, export packages containing standard javadoc doclet

[Jonathan Gibbons]

Please review this simple fix for two related aspects of the same problem:

Export the "standard doclet" used by javadoc, such that it is possible 
to derive alternative doclets, either by delegation or subtyping.

In JDK 9, javadoc has a "new" standard doclet (JEP 221), but the old one 
remains for compatibility, for the time being. Both should be exported.


Jigsaw folk:
     Please review the changes to jdk.javadoc module-info.

Build folk:
     The javadoc documentation is extended to include the documentation 
for the package containing the new StandardDoclet.
     (The API for the old StandardDoclet was never published in this 
manner; it is only included in the javadoc tool guide, which remains 
unchanged, for now.)

javadoc folk:
     The top level class for the new StandardDoclet is moved, without 
any functional change, from an internal package to the new exported package.

JBS: https://bugs.openjdk.java.net/browse/JDK-8154399
https://bugs.openjdk.java.net/browse/JDK-8159096
Webrev: http://cr.openjdk.java.net/~jjg/8154399/webrev.00

-- Jon