.NET CLI should look for restored assets using lowercase paths

[joelverhagen]

I'm working on an effort to make the package ID and version components of file paths generated by NuGet case insensitive. The most prevalent example of this is packages within the global packages folder (e.g. ~/.nuget/packages/{id}/{version}). This effort requires changes in NuGet , .NET CLI, and DotNetHost. The associated NuGet item is here:
NuGet/Home#2522

Work in .NET CLI is:

1. When CLI is looking for restored assets, use lowercase paths (ideally using NuGet APIs or the lock file path property).
2. Include the path property in the .deps.json file.

[TheRealPiotrP]

@joelverhagen I just added this to the RTM milestone.

[joelverhagen]

This is fixed is complete with @eerhardt's work in feature/msbuild and by dotnet/cli#4053, which lays the groundwork for DotNetHost.