meta: expose availability flags in derivation metadata

Currently it's not possible to determine the reason why a package is
unavailable without evaluating nixpkgs multiple times with different
settings. eg.

    nix-repl> :p android-studio.meta
    { available = false; broken = false; unfree = true; unsupported = true; ... }

The following snippet is an example that uses this information to query
the availability information of all packages in nixpkgs, giving an
overview of all the packages currently marked as broken, etc.

    { pkgs }:

    with import <nixpkgs/lib>;

    let

      mapPkgs =
        let mapPkgs' = path: f: mapAttrs (n: v:
          let result = builtins.tryEval (v ? meta); in
          if !result.success then {} else
          if isDerivation v then f (path ++ [n]) v else
          if isAttrs v && v.recurseForDerivations or false then mapPkgs' (path ++ [n]) f v else
          {}
        );
        in mapPkgs' [];

      getMeta = path: drv:
        if drv.meta ? available then
          let meta = {
            pkg = concatStringsSep "." path;
            inherit (drv.meta) broken unfree unsupported insecure;
          };
          in builtins.trace meta.pkg meta
          else {};

      metaToList = attrs: flatten (map (v: if v ? pkg then v else metaToList v) (attrValues attrs));

    in metaToList (mapPkgs getMeta pkgs)

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