bulid error in missionplanner

#### Issue details
This version of Visual Studio is unable to open the following projects. The project types may not be installed or this version of Visual Studio may not support them.
For more information on enabling these project types or otherwise migrating your assets, please see the details in the "Migration Report" displayed after clicking OK.
- Xamarin.iOS, "E:\Ashish2\MissionPlanner\ExtLibs\Xamarin\Xamarin.iOS\Xamarin.iOS.csproj"
- Xamarin.Android, "E:\Ashish2\MissionPlanner\ExtLibs\Xamarin\Xamarin.Android\Xamarin.Android.csproj"
- UsbSerialForAndroid, "E:\Ashish2\MissionPlanner\ExtLibs\UsbSerialForAndroid\UsbSerialForAndroid.csproj"

issue 2
xtLibs\UsbSerialForAndroid\UsbSerialForAndroid.csproj: The application which this project type is based on was not found. Please try this link for further information: http://go.microsoft.com/fwlink/?LinkID=299083&projecttype=10368E6C-D01B-4462-8E8B-01FC667A7035
issue 3
Severity Code Description Project File Line Suppression State
Error NuGet Package restore failed for project ExtLibs\AltitudeAngelWings: Unable to find version '2.8.0' of package 'Flurl'.
C:\Program Files (x86)\Microsoft SDKs\NuGetPackages\: Package 'Flurl.2.8.0' is not found on source 'C:\Program Files (x86)\Microsoft SDKs\NuGetPackages\'.
https://api.nuget.org/v3/index.json: Unable to load the service index for source https://api.nuget.org/v3/index.json.
An error occurred while sending the request.
The remote name could not be resolved: 'api.nuget.org'
. Please see Error List window for detailed warnings and errors.

issue 4
Severity Code Description Project File Line Suppression State
Warning NU1605 Detected package downgrade: System.IO.FileSystem from 4.3.0 to 4.0.1. Reference the package directly from the project to select a different version.
Xamarin.UWP -> Xamarin -> MAVLink -> log4net 2.0.8 -> System.Net.Sockets 4.1.0 -> runtime.win.System.Net.Sockets 4.3.0 -> System.IO.FileSystem (>= 4.3.0)
Xamarin.UWP -> Xamarin -> MAVLink -> log4net 2.0.8 -> System.IO.FileSystem (>= 4.0.1) Xamarin.UWP E:\Ashish2\MissionPlanner\ExtLibs\Xamarin\Xamarin.UWP\Xamarin.UWP.csproj 1

Start by reviewing the listed ExtLibs\Xamarin and ExtLibs\UsbSerialForAndroid project files and the Visual Studio Migration Report. Reproduce the project-load and package-restore failures, including the missing Flurl 2.8.0 and NU1605 warning. Done means the affected projects open and restore/build without the reported errors.

Written by the indexing model from the issue text.