Cache xpak metadata in ${PKGDIR}/Packages in order to minimize disk load when binaryt...

Cache xpak metadata in ${PKGDIR}/Packages in order to minimize disk load when binarytree.populate() is called.  This does not do all the keys that genpkgindex does but it will copy metadata from the existing Packages file if it appears valid.  The binarytree.populate() code is performance critical, so the MD5 will certainly have to be done elsewhere (like just after a package is built).

svn path=/main/trunk/; revision=6569