hackage2nix: don't discard platforms meta attr from postprocessing

[sternenseemann]

Distribution.Nixpkgs.Haskell.FromCabal.PostProcess defines logic that in
some cases prescribes a list of platforms. Currently this gets discarded
in hackage2nix which is a bug introduced in #506.

Before said PR, hackage2nix used a fairly elaborate logic involving Set
subtraction which was necessary because the default value would be
allKnownPlatforms. Instead of that we can now use a very simple logic:
We use whatever is set or not set by PostProcess / cabal2nix in general
and then allow whatever is yielded by that to be overwritten by the
configuration passed to hackage2nix.

[sternenseemann]

hackage-packages.nix diff:

diff --git a/pkgs/development/haskell-modules/hackage-packages.nix b/pkgs/development/haskell-modules/hackage-packages.nix
index 58f770c82c1..b820fbac18d 100644
--- a/pkgs/development/haskell-modules/hackage-packages.nix
+++ b/pkgs/development/haskell-modules/hackage-packages.nix
@@ -21936,6 +21936,7 @@ self: {
        librarySystemDepends = [ libossp_uuid ole32 ];
        description = "Support for manipulating shortcuts (.lnk files) on Windows";
        license = lib.licenses.bsd3;
+       platforms = lib.platforms.windows;
      }) {inherit (pkgs) libossp_uuid; ole32 = null;};
 
   "Wired" = callPackage
@@ -27794,6 +27795,7 @@ self: {
        libraryPkgconfigDepends = [ alsa-lib ];
        description = "Binding to the ALSA Library API (Exceptions)";
        license = lib.licenses.bsd3;
+       platforms = lib.platforms.linux;
      }) {inherit (pkgs) alsa-lib;};
 
   "alsa-gui" = callPackage
@@ -43535,6 +43537,7 @@ self: {
        librarySystemDepends = [ lxc ];
        description = "Direct Haskell bindings to LXC (Linux containers) C API";
        license = lib.licenses.bsd3;
+       platforms = lib.platforms.linux;
      }) {inherit (pkgs) lxc;};
 
   "bindings-mmap" = callPackage
@@ -91972,6 +91975,7 @@ self: {
        ];
        description = "EventStore TCP Client";
        license = lib.licenses.bsd3;
+       platforms = lib.platforms.x86_64;
        hydraPlatforms = lib.platforms.none;
        broken = true;
      }) {};
@@ -134740,6 +134744,7 @@ self: {
        libraryToolDepends = [ CoreServices ];
        description = "File/folder watching for OS X";
        license = lib.licenses.bsd3;
+       platforms = lib.platforms.darwin;
      }) {inherit (pkgs.darwin.apple_sdk.frameworks) Cocoa; 
          inherit (pkgs.darwin.apple_sdk.frameworks) CoreServices;};
 
@@ -180041,6 +180046,7 @@ self: {
        doHaddock = false;
        description = "liblzma C library and headers for use by LZMA bindings";
        license = lib.licenses.publicDomain;
+       platforms = lib.platforms.windows;
      }) {};
 
   "lzma-conduit" = callPackage
@@ -246099,6 +246105,7 @@ self: {
        ];
        description = "A software defined radio library";
        license = lib.licenses.bsd3;
+       platforms = lib.platforms.x86_64;
        hydraPlatforms = lib.platforms.none;
        broken = true;
      }) {};