CPack/NuGet: Find nuget tool on case sensitive file system

Brad Kingrequested to merge
brad.king/cmake:cpack-nuget-case into master

There is no need to use a CamelCase executable name since it will be handled gracefully on Windows anyway. This change allow support for Linux system, in particular Debian distribution where the binary is called nuget.

Ported from GitHub PR 330.

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