Lib placeholder files are not added during VS restores

[emgarten]

Place holder dlls are added to the compile and runtime sections of project entries in project.assets.json. When a project is excluded to keep it from flowing transitively these place holders are replaced with _._ to signal that no assets should be used from these projects.

Currently this works for msbuild /t:restore and dotnet.exe restore but it is not showing up when restoring from Visual Studio.

The fix for this is to add these placeholders in when creating package specs for PackageReference and project.json projects in Visual Studio.

Example:

"compile": {
    "bin/placeholder/net451/a.dll": {}
}

[emgarten]

This needs more investigation to find the impact, and the default behavior when this doesn't exist.