How do you get the current solution directory from a VSPackage?

Following is a related post in Stack Overflow (added here as a reference):

How do you get the current solution directory from a VSPackage?

Following is how you would get the current solution directory from an add-in:

_applicationObject =(DTE2)application;// retrieved from OnConnection methodstring solutionDir =System.IO.Path.GetDirectoryName(_applicationObject.Solution.FullName);

How would you do this via a VSPackage?

I'm migrating a visual studio add-in to a VSPackage as I'm intending to add some user controls that require a deeper integration with the IDE.

I found some good references on the relative merits of add-ins vs integration packages such as:http://nayyeri.net/visual-studio-addin-vs-integration-package-part-1

And some good tutorials on msdn on VSPackages such as: http://msdn.microsoft.com/en-us/library/cc138589.aspx

I haven't found a good reference yet (on msdn or otherwise) on how the higher level interfaces in add-ins (such as DTE) map to lower level interfaces in VSPackages.