Autogen directory, build directory layout in general.

[phadej]

Take a contrived example

cabal-version:       2.2
name:                autogen
version:             0.1.0.0

library
  default-language:    Haskell2010
  hs-source-dirs:      src
  build-depends:       base >=4.12 && <4.13

  exposed-modules: Autogen.Paths_autogen
  exposed-modules: Paths_autogen

with src/Autogen/Paths_autogen.hs as e.g.

module Autogen.Paths_autogen where

version :: String
version = "Hello"

On Linux the package will compile fine, but...

find dist-newstyle shows

dist-newstyle/build/x86_64-linux/ghc-8.6.5/autogen-0.1.0.0/build
dist-newstyle/build/x86_64-linux/ghc-8.6.5/autogen-0.1.0.0/build/autogen
dist-newstyle/build/x86_64-linux/ghc-8.6.5/autogen-0.1.0.0/build/autogen/Paths_autogen.hs
dist-newstyle/build/x86_64-linux/ghc-8.6.5/autogen-0.1.0.0/build/autogen/cabal_macros.h
dist-newstyle/build/x86_64-linux/ghc-8.6.5/autogen-0.1.0.0/build/Paths_autogen.dyn_hi
dist-newstyle/build/x86_64-linux/ghc-8.6.5/autogen-0.1.0.0/build/Paths_autogen.o
dist-newstyle/build/x86_64-linux/ghc-8.6.5/autogen-0.1.0.0/build/libHSautogen-0.1.0.0-inplace-ghc8.6.5.so
dist-newstyle/build/x86_64-linux/ghc-8.6.5/autogen-0.1.0.0/build/Paths_autogen.dyn_o
dist-newstyle/build/x86_64-linux/ghc-8.6.5/autogen-0.1.0.0/build/Autogen
dist-newstyle/build/x86_64-linux/ghc-8.6.5/autogen-0.1.0.0/build/Autogen/Paths_autogen.dyn_hi
dist-newstyle/build/x86_64-linux/ghc-8.6.5/autogen-0.1.0.0/build/Autogen/Paths_autogen.o
dist-newstyle/build/x86_64-linux/ghc-8.6.5/autogen-0.1.0.0/build/Autogen/Paths_autogen.dyn_o
dist-newstyle/build/x86_64-linux/ghc-8.6.5/autogen-0.1.0.0/build/Autogen/Paths_autogen.hi
dist-newstyle/build/x86_64-linux/ghc-8.6.5/autogen-0.1.0.0/build/Paths_autogen.hi
dist-newstyle/build/x86_64-linux/ghc-8.6.5/autogen-0.1.0.0/build/libHSautogen-0.1.0.0-inplace.a

I don't think that autogen and Autogen would cause problems on case-insensitive filesystem & platforms (e.g. Windows, macOS), but I don't like the lack of hygiene anyway.

Maybe this would be easier to refactor after #6667 is started.