Arrange stackage.nix in subdirectories

[rvl]

stackage.nix is almost entirely auto-generated, but if it were organised into subdirectories then the main directory listing wouldn't break github.

I propose:

• stackage.nix/
  • lts/
    • ...
    • lts-11.19.nix
    • ...
  • nightly/
    • ...
    • nightly-2019-02-09.nix
    • ...
  • README.org
  • ltss.nix
  • nightlies.nix
  • default.nix
  • lts-haskell (submodule)
  • stackage-nightly (submodule)