The PackageMojo should install XAR dependencies starting with the most depended upon ones

Use-Case:

• I have a XAR A holding 10 pages including MySpace.MyPage.
• I have a XAR B holding some pages including MySpace.MyPage, that it overwrites.
• XAR B depends on XAR A.

Currently, we have nothing to enforce the installation of the XAR A before the XAR B on the PackageMojo. Results: in some cases, you have MySpace.MyPage from XAR A, and in some other case, you end up with MySpace.MyPage from XAR B.

If we install the XAR dependencies in a smart order (dependencies first), we would be sure that the XAR B would be the last installed .

It is already how the ImportMojo works (see: https://github.com/xwiki/xwiki-platform/blob/master/xwiki-platform-tools/xwiki-platform-tool-packager-plugin/src/main/java/com/xpn/xwiki/tool/backup/ImportMojo.java#L173-173).