Lay down the foundation for packaging the .NET echosystem

- fetchNuGet can fetch binaries from nuget servers
- buildDotnetPackage can build .NET packages using mono/xbuild
  - Places nuget & paket as they would clash with nix
  - Patch project files because F# targets are expected to be found in
    the mono directory (and we know that's not going to happen on nix)
  - Find DLLs that were copied from buildInputs and replace by symlink
    for sharing
  - Export produced DLL via the pkg-config mechanism
  - Create wrappers for produced EXEs
- Repackaged this new infrastructure: keepass, monodevelop
- Newly packaged: ExtCore, UnionArgParser, FSharp.Data, Paket, and a
  bunch more..

This is a combination of 73 commits.

pkgs/build-support/dotnetbuildhelpers/create-pkg-config-for-dll.sh

@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+
+targetDir="$1"
+dllFullPath="$2"
+
+dllVersion="$(monodis --assembly "$dllFullPath" | grep ^Version: | cut -f 2 -d : | xargs)"
+[ -z "$dllVersion" ] && echo "Defaulting dllVersion to 0.0.0" && dllVersion="0.0.0"
+dllFileName="$(basename $dllFullPath)"
+dllRootName="$(basename -s .dll $dllFileName)"
+targetPcFile="$targetDir"/"$dllRootName".pc
+
+mkdir -p "$targetDir"
+
+cat > $targetPcFile << EOF
+Libraries=$dllFullPath
+
+Name: $dllRootName
+Description: $dllRootName
+Version: $dllVersion
+Libs: -r:$dllFileName
+EOF
+
+echo "Created $targetPcFile"