dotnet build doesn't resolve package references correctly for old style projects

### Steps to reproduce

Dotnet build does not resolve package references in old style projects, but restore seems to be done fine. I tried also several variations like dotnet msbuild /t:restore;build etc. All lead to the same build errors.

[PackageReferenceBugDemo.zip](https://github.com/microsoft/msbuild/files/4448967/PackageReferenceBugDemo.zip)

Project file
```xml

Debug
AnyCPU
{C27219F1-6C86-4D9C-B9C3-4B01B5D9879B}
Library
Properties
PackageReferenceBugDemo
PackageReferenceBugDemo
v4.8
512
true


true
full
false
bin\Debug\
DEBUG;TRACE
prompt
4


pdbonly
true
bin\Release\
TRACE
prompt
4

```

Directory contents:
```
see attachment
```

Command line
```
dotnet build
```
### Expected behavior

The build runs fine

### Actual behavior

The nunit package reference doesn't get resolved

### Environment data
dotnet msbuild //version
```
Microsoft (R)-Build-Engine, Version 16.4.0+e901037fe f▒r .NET Core
Copyright (C) Microsoft Corporation. Alle Rechte vorbehalten.
```