Unload packrat after calling it to check project context

This fixes a bug whereby RStudio would _always_ load packrat into the current session, even if the project was not using packrat.

NOTE: I feel a little bit uncomfortable about this change; I wonder if instead we should have our own shims that check the 'packrat status' of a project without calling into packrat itself (which forces a packrat load)
would this want to be .rs.forceUnloadPackage?