[onnxruntime] Fix use of Eigen3, remove NuGet executable usage

[luncliff]

Changes

• Remove NuGet usage in portfile.cmake. It is no longer used.
• Fix header search of Eigen3 in Linux build

References

• [xnnpack,onnxruntime] Use later version of XNNPack, ONNXRuntime #151
• CI: Use AWS S3 binary caching in CircleCI build #152

Triplet Support

• x64-windows
• x64-linux
• x64-osx

Configuration

"vcpkg-configuration.json" changes for the release.

{
    "registries": [
        {
            "kind": "git",
            "repository": "https://github.com/luncliff/vcpkg-registry",
            "packages": [
                {
                    "name": "onnxruntime",
                    "features": [
                        "training",
                        "xnnpack"
                    ]
                }
            ],
            "baseline": "..."
        }
    ]
}