ResolvePackageDependencies Task does not work on case-sensitive file systems

[brthor]

The issue is that nuget lower-cases all file paths in the nuget cache, and ResolvePackageDependencies should do the same.

https://github.com/dotnet/sdk/blob/master/src/Tasks/Microsoft.DotNet.Core.Build.Tasks/ResolvePackageDependencies.cs#L329

[brthor]

@eerhardt @natidea