Create target platform for mirrored parts of Central / 3rd Party Extras

As discussed in JBDS-2486 ( https://issues.jboss.org/browse/JBDS-2486?focusedCommentId=12770186&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-12770186 ) we need to produce a target file for use with Central.

Goal here would be to build the Extras site (used by Central) as a TP update site rather than a composite of mirrors, which would provide us with a manifest of exactly which versions of these duped IUs were to be contained in the Extras site. If something bad happens, we can add duplicate versions of IUs to the TP and know exactly why we include both Jetty 8.1.3 and 8.1.9 (for example) or two different spins of WindowBuilder. This would mean it wouldn't matter if we filtered content out of the mirrors, because we'd be handling the filtering in a single place (extras.target) rather than multiple build.xml files.

In future, we would:

• mirror the individual upstream projects into /updates/requirements/<project> [on dl.jb.org]
• mirror the individual upstream projects into /updates/requirements/<project> [on www.qa]

• produce an aggregate of the stuff that Central needs, and publish that instead of the composite [on dl.jb.org]
• produce an aggregate of the stuff that Central needs, and publish that instead of the composite [on www.qa]
• produce an aggregate of the stuff that Central needs, and publish that instead of the composite [on ds.jb.com]

• add a whole new aggregate for old + new content