folly: expose fmt and boost libs

This allows for people consuming these
packages to inherit the same libaries.

This is needed because the folly-config.cmake
will use `find_package` on these dependencies,
thus downstream packages should avoid potential
abi issues.

pkgs/development/libraries/folly/default.nix

···
   NIX_CFLAGS_COMPILE = [ "-DFOLLY_MOBILE=${if follyMobile then "1" else "0"}" "-fpermissive" ];
   cmakeFlags = [ "-DBUILD_SHARED_LIBS=ON" ];
 
+  # folly-config.cmake, will `find_package` these, thus there should be
+  # a way to ensure abi compatibility.
+  passthru = {
+    inherit boost;
+    fmt = fmt_8;
+  };
+
   meta = with lib; {
     description = "An open-source C++ library developed and used at Facebook";
     homepage = "https://github.com/facebook/folly";