Properties delivered in a NuGet package referenced via packages.config are not reevaluated after restore

This issue arises when trying to deliver MSBuild targets/properties in a NuGet package (which I'll call the *targets package*). The targets package uses the new csproj format and is multi-targeted. In my case, I am targeting `netstandard2.0;net461`.

The targets package delivers a `props` file which contains the following property:

```

example property

```

and a `targets` file which contains the following target:

```

```

When the targets package is packed to a NuGet package and then installed into a project using a 'classic' csproj (which I'll call the host package), these lines are added - as expected - to the csproj:

```

[...]

This project references NuGet package(s) that are missing on this computer. Use NuGet Package Restore to download them. For more information, see http://go.microsoft.com/fwlink/?LinkID=322105. The missing file is {0}.

```
However, when building the host package, the properties are not evaluated. The build prints the incorrect output `The example property is `.

### Steps to reproduce

* Clone https://github.com/rossng/PropsBugDemonstrator
* Open the solution in Visual Studio (tested with 2019/16.0.0 preview 3.0)
* Build `PropsProject` to produce the targets package `PropsProject\bin\Debug\PropsProject.1.0.0.nupkg`
* Copy the NuGet package to your local repository
* Restore packages for the `PropsBugDemonstrator` project
* Rebuild the `PropsBugDemonstrator` project

### Expected behaviour

During the build, the `TestTarget` defined in the targets package is executed and prints `The example property is example property`.

### Actual behaviour

During the build, the `TestTarget` defined in the targets package is executed and prints `The example property is `.

Log excerpt:

```
1>Target "TestTarget: (TargetId:3)" in file "C:\Users\Ross Gardiner\Documents\Projects\PropsBugDemonstrator\packages\PropsProject.1.0.0\build\PropsProject.targets" from project "C:\Users\Ross Gardiner\Documents\Projects\PropsBugDemonstrator\PropsBugDemonstrator\PropsBugDemonstrator.csproj" (target "BeforeBuild" depends on it):
1>Task "Message" (TaskId:4)
1> Task Parameter:Text=The example property is (TaskId:4)
1> Task Parameter:Importance=High (TaskId:4)
1> The example property is (TaskId:4)
1>Done executing task "Message". (TaskId:4)
```

### Other notes

If you can't reproduce the issue, try deleting all `packages`, `bin` and `obj` folders before building.

The issue does not arise if using the new csproj format for `PropsBugDemonstrator`. In the 'real' instance of this bug I do not currently have the option to switch to the new format.

I am unable to reproduce the issue using the command line interface to MSBuild. Steps to try this are (starting from repository root):

```
cd PropsBugDemonstrator
nuget restore -SolutionDirectory ..
msbuild PropsBugDemonstrator.csproj
```

### Environment data
`msbuild /version` output:

```
Microsoft (R) Build Engine version 16.0.360-preview+g9781d96883 for .NET Framework
Copyright (C) Microsoft Corporation. All rights reserved.

16.0.360.33175
```

and

```
Microsoft (R) Build Engine version 15.9.21+g9802d43bc3 for .NET Framework
Copyright (C) Microsoft Corporation. All rights reserved.

15.9.21.664
```

OS info:

Windows 10 Pro for Workstations 1809, 17763.316

If applicable, version of the tool that invokes MSBuild (Visual Studio, dotnet CLI, etc):

* Visual Studio 16.0.0 Preview 3.0
* Visual Studio 15.9.7