add volatile sources support in libapt-pkg

add volatile sources support in libapt-pkg

Sources are usually defined in sources.list (and co) and are pretty
stable, but once in a while a frontend might want to add an additional
"source" like a local .deb file to install this package (No support for
'real' sources being added this way as this is a multistep process).

We had a hack in place to allow apt-get and apt to pull this of for a
short while now, but other frontends are either left in the cold by this
and/or the code for it looks dirty with FIXMEs plastering it and has on
top of this also some problems (like including these 'volatile' sources
in the srcpkgcache.bin file).

So the biggest part in this commit is actually the rewrite of the cache
generation as it is now potentially a three step process. The biggest
problem with adding support now through is that this makes a bunch of
previously mostly unusable by externs and therefore hidden classes
public, so a bit of further tuneing on this now public API is in order…