mkNugetSource: remove mono from build closure

A directory full of *.nupkg files is a valid nuget source. We do not need mono
and the Nuget command line tool to create this structure. This has two
advantages:

- Nuget is currently broken due to a kernel bug affecting mono (#229476).
  Replacing the mkNugetSource implementation allows affected users on 6.1+
  kernels compile .NET core packages again.
- It removes mono from the build closure of .NET core packages. .NET core
  builds should not depend on .NET framework tools like mono.

There is no equivalent of the `nuget init` command in .NET core. The closest
command is `dotnet nuget push`, which just copies the *.nupkg files around
anyway, just like this PR does with `cp`.

`nuget init` used to extract the *.nuspec files from the nupkgs, this new
implementation doesn't. .NET core doesn't care, but it makes the license
extraction more difficult. What was previously done with find/grep/xml2 is now
a python script (extract-licenses-from-nupkgs.py).

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