fix building in VS with PDB conversions

[brettfo]

There were two issues with building in Visual Studio:

1. Converting the portable PDB to a Windows-style PDB was failing because the command-line build explicitly sets the %NUGET_PACKAGES% environment variable to <repo-root>\packages, but that variable wasn't set when VisualFSharp.sln was open in VS so it was falling back to the common location of %USERPROFILE%\.nuget\packages which doesn't work. The fix for that is to set the fallback to <repo-root>\packages.
2. The property pages project explicitly sets its $(OutputPath) variable which means the HACK_CopyOutputsToTheProperLocation target was being unnecessarily run which was resulting in incorrect PDBs and locked files.

[dsyme]

We might need this on a few more projects

[brettfo]

At this point we shouldn't. The only projects picking up the path copying hack from FSharp.Directory.Build.targets are projects converted to the SDK where the copy hack is necessary, and this property pages project. All other projects in our repo directly write outputs to release\net40\bin, but because we ultimately depend on $(TargetFramework) from SDK projects to determine release\net40\bin vs. release\coreclr\bin, we have to copy files after the build has completed.

tl;dr - This is the only project file that can properly set $(OutputPath) and that pulls in FSharp.Directory.Build.targets which defaults to the copy hack.

[dsyme]

OK, understood, thanks. I thought it was about the Copy File conflicts.