paket pack does not support AppendTargetFrameworkToOutputPath=false

Description

Running paket pack on a project that has AppendTargetFrameworkToOutputPath set to false tries to look up the output assembly in ${OutputPath}\${TargetFramework} whereas the assembly is compiled into ${OutputPath}

Repro steps

Please provide the steps required to reproduce the problem

1. Create a .NET Core class lib and set AppendTargetFrameworkToOutputPath to false.
2. Create paket.template file
3. Run paket pack

Expected behavior

Packing works.

Actual behavior

Loading /Users/agross/gw/grossweber/ops/source/Ops.Contracts/Ops.Contracts.csproj
Loading assembly metadata for /Users/agross/gw/grossweber/ops/build/bin/ops-host/bin/net5.0/Ops.Contracts.dll
Total time taken: 0 milliseconds
 - Runtime: 1 second
Paket failed with
-> DirectoryNotFoundException: Could not find a part of the path '/Users/agross/gw/grossweber/ops/build/bin/ops-host/bin/net5.0/Ops.Contracts.dll'.
   StackTrace:
   ...

Reproduce the issue with a .NET Core class library whose AppendTargetFrameworkToOutputPath is false, a paket.template file, and paket pack. Start by tracing paket pack's output-assembly lookup; done means it finds the assembly directly under OutputPath and packing succeeds without the DirectoryNotFoundException.