Create a CPack generator that uses `nuget.exe` to create packages:
https://docs.microsoft.com/en-us/nuget/what-is-nuget
NuGet packages could be easily produced from a `*.nuspec` file (running
`nuget pack` in the directory w/ the spec file). The spec filename does
not affect the result `*.nupkg` name -- only `id` and `version` elements
of the spec are used (by NuGet).
Some implementation details:
* Minimize C++ code -- use CMake script do to the job. It just let the
base class (`cmCPackGenerator`) to preinstall everything to a temp
directory, render the spec file and run `nuget pack` in it, harvesting
`*.nupkg` files...;
* Ignore package name (and use default paths) prepared by the base class
(only `CPACK_TEMPORARY_DIRECTORY` is important) -- final package
filename is a responsibility of NuGet, so after generation just scan the
temp directory for the result `*.nupkg` file(s) and update
`packageFileNames` data-member of the generator;
* The generator supports _all-in-one_ (default), _one-group-per-package_
and _one-component-per-package_ modes.
