Fresh new Xamarin.Forms application created from template yields a ton of errors

I created a new Android and iOS/NETStandard Xamarin.Forms application with Visual Studio 2017 15.5.4 and immediately got an exclamation mark on Dependencies node in Common project. Trying to build the solution I got like three errors for start. They were saying something along

error : Assets file '[APPPATH]\obj\project.assets.json' not found. Run a NuGet package restore to generate this file.

Since I had no idea what that message wanted to tell me I went checking for references and I quickly saw that nor Android nor iOS had any NuGet reference present albeit the packages were listed as installed. So, no packages for some reason. I quickly dismissed the possibility of bad "new Xamarin.Forms project" template - Internet would be screaming in such case.
After tinkering a bit, I saw that Package Manager window was saying that it can't find a reference to NuGet Source for one of my sources. Which was correct, but I wasn't using that source at all.

It turns out that once disabling that (faulty) source, NuGet was able to restore the packages and project was building.
Actually it was a combination of two problems combined - one was that missing source that prevented NuGet to download packages and the other problem was that the required NuGet packages weren't cached locally on my computer - if they were I believe it would work regardless of the faulty source.

Troubleshooting Live Unit Test build failure

It happens that I have a .NET 4.6.2 solution including Unit Test projects that builds just fine. However, when starting Live Unit Test it would complain build completed (failed) for some reason. Which is weird since the solution builds fine.

I started searching for the reason of this mysterious build failure. First thing to do is to enable verbose Live Unit Test logging: Tools/Options.../Live Unit Testing -> Logging Level. Once set to verbose it become obvious that Live Unit Test BuildManager had problems with some referenced third party assemblies, specifically (in my case) with NUnit and Moq. Errors were like this:

[09:02:50.728 Verbose] - BuildManager - MY_TEST_PROJECT_PATH\SomeFile.cs(1,7): error CS0246: The type or namespace name 'NUnit' could not be found (are you missing a using directive or an assembly reference?)

But why wouldn't it find these Assemblies where VS has no problem finding them. Note: I added these two (and other) assemblies through NuGet.
I went looking where the problematic references actually point (Solution Explorer, Test Project, References) and saw something odd. Instead of pointing to a some path under NuGet Packages directory they were pointing to build output directory. At some point in the past VS decided that it has to change the path to the assemblies for some reason. Probably it couldn't find them at original location and decided to find the only ones it could - ones from the recent build in the build output directory. Which is a bad move from Visual Studio and one that most certainly causes problems.

At the end the solution was rather easy once I discovered the reason. I simply upgraded NuGet packages to a newer version. Reinstall would also work.