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/development/compilers/fsharp/default.nix

@@ -1,4 +1,6 @@
-{ stdenv, fetchurl, mono, pkgconfig, autoconf, automake, which }:
+# Temporaririly avoid dependency on dotnetbuildhelpers to avoid rebuilding many times while working on it
+
+{ stdenv, fetchurl, mono, pkgconfig, dotnetbuildhelpers, autoconf, automake, which }:
 
 stdenv.mkDerivation rec {
   name = "fsharp-${version}";
@@ -9,7 +11,7 @@ stdenv.mkDerivation rec {
     sha256 = "16kqgdx0y0lmxv59mc4g7l5ll60nixg5b8bg07vxfnqrf7i6dffd";
   };
 
-  buildInputs = [ mono pkgconfig autoconf automake which ];
+  buildInputs = [ mono pkgconfig dotnetbuildhelpers autoconf automake which ];
   configurePhase = ''
     substituteInPlace ./autogen.sh --replace "/usr/bin/env sh" "/bin/sh"
     ./autogen.sh --prefix $out
@@ -23,6 +25,10 @@ stdenv.mkDerivation rec {
     substituteInPlace $out/bin/fsharpiAnyCpu --replace " mono " " ${mono}/bin/mono "
     ln -s $out/bin/fsharpc $out/bin/fsc
     ln -s $out/bin/fsharpi $out/bin/fsi
+    for dll in "$out/lib/mono/4.5"/FSharp*.dll
+    do
+      create-pkg-config-for-dll.sh "$out/lib/pkgconfig" "$dll"
+    done
   '';
 
   # To fix this error when running: