RFR: 8195795: Organize javadoc output files by module/package, not just package

[Jonathan Gibbons]

Please review changes, including a couple of small build changes, to 
reorganize the generated
documentation into per-module directories.

Build folk: the changes are just to move the generated module graph 
images into the new hierarchy.

Javadoc folk: the changes are mostly simple, with most of the "magic" 
happening in the DocPaths factory class, creating old-style or new-style 
paths as appropriate. Care is taken in DocFilesHandlerImpl, where 
DocPath objects are used for input, and have to work in conjunction with 
the module Locations.  Other than than, some factory methods moved from 
DocPath to DocPaths, and most factory methods were changed from static 
to instance methods, to be able to take the interim 
backwards-compatibility option into account.

This work leverages JDK-8195796, to reduce the size of relative URLs in 
generated docs.

JBS: https://bugs.openjdk.java.net/browse/JDK-8195795
CSR: https://bugs.openjdk.java.net/browse/JDK-8196112
Webrev: http://cr.openjdk.java.net/~jjg/8195795/webrev.00/

-- Jon