Remove driver and session from PackageGraph

[mehmetf]

What
I would like to remove the presence of AnalysisDriver and Session from PackageGraph.

Why
PackageGraph is available to many model objects and it makes good sense to reduce the available API surface.

It is preferred that PackageBuilder populates all necessary information for dartdoc to function during build phase rather than relying on model objects to lazily get them from AnalysisSession. This allows us to handle situations where an AnalysisSession is not available such as constructing ResolvedLibrary objects from summary data.

[jcollins-g]

LGTM

This is going in the general direction dartdoc has been moving for a while. Originally, all elements were lazily fetched and the fact we still have some support for that is vestigal at this point. Thanks for the cleanup!