Make package ID and version comparison case insensitive

[joelverhagen]

The server already does this and version comparison in memory is case insensitive. We've also done a pretty good job about making ID comparison in memory a case insensitive operation.

However, there are places where ID comparisons are case sensitive. Namely, code that interacts with the file system. For example, when a local source is a directory of .nupkgs, the ID comparisons are case sensitive.

The plan forward here is:

• Add a property to the project.lock.json file so that each package libraries (under the libraries node) has a path. The value of this property should be {id}/{version} component of a path which can be used along with the global packages folder to get the absolute path to package assets.
• Add a switch to NuGet to support restoring the original ID and version format. This is for backwards compatibility (with Add --legacy-packages-directory to Xplat restore NuGet.Client#814).
• Update the .NET CLI to write this property to the .deps.json file.
• Update the .NET CLI to write "hashPath" to the .deps.json file.
• Update the DotNetHost to prefer the path property over the concatenation of ID and version when reading the .deps.json file and use the "hashPath" property when available.
• Revert the hack which always lowercases the package ID and version in the .deps.json file.
• Work with owners of DOTNET_HOSTING_OPTIMIZATION_CACHE to ensure the new cache is built with the proper format.

[joelverhagen]

Related: #2383

[joelverhagen]

@yishaigalatzer, we should consider this for CLI RTM since it would necessitate a major change to the global packages folder -- making all IDs and versions lowercase. This change could be breaking in some Mac and Linux scenarios and would necessitate that packages are downloaded or fetched from the HTTP cache again.

[yishaigalatzer]

That's a pretty major breaking change. I would first try to find an alternative but if we can't we should have an urgent design meeting with the CLI folks