Paket pack not handling "SourceDir" correctly in *.csproj file

Description

If the "OutputPath" has $(SolutionDir) for then Paket pack has a DirectoryNotFoundException. It appears to be concatenating the current working directory to the actual output directory, instead of using the true Visual Studio solution directory, and then concatenate the output directory.

Broken:
$(SolutionDir)buildoutput\Debug</OutputPath>

Works:
....\buildoutput\Debug</OutputPath>

Repro steps

Please provide the steps required to reproduce the problem

1. Change the definition of the "OutputPath" to use $(SolutionDir) instead of a relative path to the directory where the solution file actually exists.

2. Build the solution.

3. CD to the directory that has the *.csproj file and paket.template (type project) and run "paket pack . --version 1.2.3.4 --verbose --template .\paket.template" Then the exception should be thrown. The path to the DLL it is looking to pack should exist under the directory the *.csproj file exists but really exists in a directory relative to the actual Visual Studio solution directory.

If possible then please create a git repository with a repro sample or attach a zip to the issue.

Expected behavior

I would expect Paket to determine the directory location of the solution file and use that instead of the directory the "paket pack..." command is being run from as the $(SolutionDir).

Actual behavior

Seeing a DirectoryNotFoundException and the path indicated is expected under the directory the *.csproj file exists in and is not looking for the *.dll to be included relative to the solution file.

Known workarounds

Manually change the $(SolutionDir) in the OutputPath to a relative path.

Reproduce the failure with a .csproj whose OutputPath uses $(SolutionDir), then run paket pack . --version 1.2.3.4 --verbose --template .\paket.template from the project directory. Trace how paket pack resolves the DLL path and compare it with the solution directory; done means the existing DLL is found and packaged without a DirectoryNotFoundException.