Fixed running Windows-packed projects on Linux

The project.json generated on Windows makes the webroot path relative to the pack folder, but does so using Path.DirectorySeparatorChar which is '\'. This path throws when runnig the project under Linux.

This PR changes the two paths in JSON files I found ('webroot' in project.json and 'packages' in global.json) to consistently use forward slashes.
I think it's better to add a name for the last argument:
`PathUtility.GetRelativePath(targetProjectJson, targetWebRootPath, separator: '/');`