Graph load issue evaluating in parallel

```
git clone https://github.com/OrchardCMS/OrchardCore.git
cd OrchardCore
git checkout b74a906d15eba7de4c637e80fcc5decbc13f593f
msbuild /t:Restore /v:m /m
```
[dirs.zip](https://github.com/Microsoft/msbuild/files/2507851/dirs.zip)

```C#
var path = @"";
var graph = new ProjectGraph(path);
```

Note: To create the dirs.proj I did `msbuild /bl` on the `.sln` file, and thanks to @ccastanedaucf `OrchardCore.sln.metaproj` is embedded in the binary log.

Exception thrown:
```
System.Collections.Generic.KeyNotFoundException: 'The given key was not present in the dictionary.'
```

Does not repro before db2d625ff804d36f3dce3ef4821417e5b83b8473.