Upgrade 6.4.2 to 7.0.0 fails due to invalid target?
## 🐛 Bug Report

When I try to upgrade my solutions from 6.4.2 to 7.0.0 I always get a error when nuget tries to upgrade my UnitTest project. Based on the dependencies I would assume it should work because I can't find any impactful changes on the requirements. In the output I only get a hint to my UnitTest project, as you can see here: 

```
Restoring packages for F:\Repos\BreathCompanion\BreathCompanion\BreathCompanion\BreathCompanion.TestBase\BreathCompanion.TestBase.csproj...
Restoring packages for F:\Repos\BreathCompanion\BreathCompanion\BreathCompanion\BreathCompanion.Core\BreathCompanion.Core.csproj...
Restoring packages for F:\Repos\BreathCompanion\BreathCompanion\BreathCompanion\BreathCompanion.iOS\BreathCompanion.iOS.csproj...
Restoring packages for F:\Repos\BreathCompanion\BreathCompanion\BreathCompanion\BreathCompanion.Droid\BreathCompanion.Droid.csproj...


Attempting to gather dependency information for multiple packages with respect to project 'Tests\BreathCompanion.UnitTests', targeting '.NETFramework,Version=v4.7'
Gathering dependency information took 19,03 sec
Attempting to resolve dependencies for multiple packages.
Resolving dependency information took 0 ms
Resolving actions install multiple packages
Removed package 'MvvmCross.Forms 6.4.2' from 'packages.config'
Successfully uninstalled 'MvvmCross.Forms 6.4.2' from BreathCompanion.UnitTests
Removed package 'MvvmCross.Plugin.Messenger 6.4.2' from 'packages.config'
Successfully uninstalled 'MvvmCross.Plugin.Messenger 6.4.2' from BreathCompanion.UnitTests
Removed package 'MvvmCross 6.4.2' from 'packages.config'
Successfully uninstalled 'MvvmCross 6.4.2' from BreathCompanion.UnitTests
Adding package 'MvvmCross.7.0.0' to folder 'F:\Repos\BreathCompanion\BreathCompanion\packages'
Added package 'MvvmCross.7.0.0' to folder 'F:\Repos\BreathCompanion\BreathCompanion\packages'
Install failed. Rolling back...
Package 'MvvmCross 7.0.0' does not exist in project 'BreathCompanion.UnitTests'
Package 'MvvmCross 6.4.2' already exists in folder 'F:\Repos\BreathCompanion\BreathCompanion\packages'
Added package 'MvvmCross 6.4.2' to 'packages.config'
Package 'MvvmCross.Plugin.Messenger 6.4.2' already exists in folder 'F:\Repos\BreathCompanion\BreathCompanion\packages'
Added package 'MvvmCross.Plugin.Messenger 6.4.2' to 'packages.config'
Package 'MvvmCross.Forms 6.4.2' already exists in folder 'F:\Repos\BreathCompanion\BreathCompanion\packages'
Added package 'MvvmCross.Forms 6.4.2' to 'packages.config'
Removing package 'MvvmCross 7.0.0' from folder 'F:\Repos\BreathCompanion\BreathCompanion\packages'
Removed package 'MvvmCross 7.0.0' from folder 'F:\Repos\BreathCompanion\BreathCompanion\packages'
Executing nuget actions took 7,6 sec
Failed to add reference to 'MvvmCross'.
  The project targets '.NETFramework' while the file reference targets '.NETCore'. This is not a supported scenario.
Time Elapsed: 00:00:37.3572452
========== Finished ==========


```

### Expected behavior

Upgrade should work

### Reproduction steps

Unknown, guess it will always happen on a .NETFramework 4.7 project?

### Configuration

**Version:** 6.4.2 to 7.0.0

**Platform:** 
- [x] :iphone: iOS
- [x] :robot: Android
- [ ] :checkered_flag: WPF
- [ ] :earth_americas: UWP
- [ ] :apple: MacOS
- [ ] :tv: tvOS
- [x] :monkey: Xamarin.Forms
