With "the DXA 1.7 Nuget package" I'm guessing you mean "SDL Digital eXperience Accelerator Framework for SDL Web 8", which when you inspect the contents of the package, does contain the
SDL.Web.Tridion.dll in its
The reason it is not referenced from a Web project, is because of clarity on dependencies. a Web project (see https://github.com/sdl/dxa-web-application-dotnet/blob/release/1.7/Site/DxaWebApp.csproj) references
Sdl.Web.Mvc.dll, and the latter has a reference to
SDL.Web.Tridion.dll. The web project itself should never have a direct reference to
SDL.Web.Tridion.dll (since in DXA we abstract the Tridion API away to make it a more standard MVC application, easier to understand for Web developers, so they don't need Tridion API knowledge).
The cleanest and best way to include the required DXA framework libs in your project can again be found in our example project: https://github.com/sdl/dxa-web-application-dotnet/blob/release/1.7/Site/DxaWebApp.csproj, it contains a target BeforeBuild:
<CallTarget Targets="CopyDxaFrameworkLibsToOutput" />
The NuGet package will contains further details about that target and what it does, but simply put, it copies the
SDL.Web.Tridion.dll DLL to the
bin directory of your project before build.
All of this is also covered in the documentation under the topic "Setting up a new ASP.NET MVC Web application".
note: I'm wondering once you are done with your new DXA project, how "different" it will be compared to the example project which DXA already provides you, would gladly receive some feedback as to why you think the example is not usable for you.