libapt-pkg v2 to v3 incorporates several source-incompatible changes
that people need to be aware of.. Many of these changes are done so that
most old source will continue to function, but perhaps at reduced
functionality.

* pkgDepCache is no longer self initializing, you have to call the Init
  method separately after constructing it. Users of pkgCacheFile do not
  need to worry about this
* GetCandidateVer/etc is gone from the pkgCache. It exists only in the 
  DepCache and is just an inline around the new Policy class
* TargetVer/TargetDist have been eliminated. Nothing should have been using
  these.
* There is a policy class. The v0 policy engine which has been used since
  APT 0.0.0 is instantiated by the DepCache by default. However pkgCacheFile 
  constructs and initializes the new v4 engine. People accessing GetCandidate
  version outside of a CacheFile/DepCache will need to instantiate and 
  initialize a policy engine on their own.
* All byte counters are now doubles to advoid 4G wraparound. The compiler 
  should generate warnings on any incorrect use of these.
* The PriorityType/CompType/DepType functions have been moved out of the 
  iterators and into generate static functions of pkgCache - inline stubs
  are left in the iterators. 
* The deb dependency element parser has been made into a static function
  of the list parser and enhanced to optionally understand architecture 
  restrictions.
* TagSections no longer include the trailing \n. This means that the 
  Offset/Length of a package record in the version structure also does not
  include the trailing \n.
* GenCaches::SelectFile accepts a site parameter now too.
* Global version compare functions are gone. If you 
    #define APT_COMPATIBILITY 1
  They will come back as they were before. Code should be updated to 
  reference the compare functions to the VersioningSystem (VS) referenced
  by the Cache or _system structures.
* Initialization is now two stage (define APT_COMPATIBILITY..) The first
  stage, pkgInitConfig is called before commandline parsing, and
  pkgInitSystem is called after. This gives the user the opportunity to
  override default settings from the config files before startup has been
  finalized.
* pkgSourceList has been gutted. All the junk that was in there before is
  cleaned up and put in the pkgIndexFile class. There is very little API
  correspondence here..
* pkgMakeStatusCacheMem is gone, pkgMakeStatusCache does the same thing if
  you set the AllowMem flag. Also, you can get a copy of the map used to
  store the cache to advoid having to remap it in the calling code. A bunch
  of other cache related functions are gone, but nobody should have been using
  them in the first place!
* Downloading the 'Package' and 'Source' index files is different, use
  the GetIndexes call in SourceList.
* SourceRecords::Parser::Source is gone, replaced with Index which does 
  much the same thing.
* DynamicMap has changed slightly, nobody should care
* pkgMakeOnlyStatusCache exists, which creates a really small cache that
  only contains the status file, and in memory.
* The pkgRecords stuff is changed to abstract through the index file list
  (should be transparent largely)
* Locking is handled differently, there is no dpkg lock class, the _system
  class provides Lock/UnLock methods
* pkgDepCache is not a subclass of pkgCache, it aggregates it now. Some
  compatibility functions are provided that make this transition fairly 
  easy.
* The following functions have had minor argument changes:
   -   pkgSimulate(pkgDepCache &Cache);
   +   pkgSimulate(pkgDepCache *Cache);
   
   -   pkgProblemResolver(pkgDepCache &Cache);
   +   pkgProblemResolver(pkgDepCache *Cache);
   
   -   pkgDepCache(MMap &Map,Policy *Plcy = 0);
   +   pkgDepCache(pkgCache *Cache,Policy *Plcy = 0);
   
   -   pkgOrderList(pkgDepCache &Cache);
   +   pkgOrderList(pkgDepCache *Cache);
   
   -   pkgPackageManager(pkgDepCache &Cache);
   +   pkgPackageManager(pkgDepCache *Cache);
   
   -   pkgCache(MMap &Map,bool DoMap = true);
   +   pkgCache(MMap *Map,bool DoMap = true);
   
   -   pkgCacheGenerator(DynamicMMap &Map,OpProgress &Progress);
   +   pkgCacheGenerator(DynamicMMap *Map,OpProgress *Progress);
   
   -   pkgTagFile(FileFd &F,unsigned long Size = 32*1024);
   +   pkgTagFile(FileFd *F,unsigned long Size = 32*1024);
   
* Configuration class is const-correct  
* The legacy ability to create a PkgFileIterator that started at Begin
  is gone, everyone should be using FileBegin().
* A new dependency relation called obsoletes that is similar to conflicts.