* pulled commonly useful parts out of PackageWriterImpl into
WriterImplBase
* moved CachedStringTable and related methods into a separate class,
StringCache, in order to support having more than one string cache
per package file
* made package attribute section use a string cache, too, as that's
going to be very useful for repositories
* instead of writing package attributes directly, we now collect
corresponding PackageAttributes and write those later
* adjusted package reader accordingly
git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@40376 a95241bf-73f2-0310-859d-f6bbb57e9c96
