cabal new-build chokes on executable autogen

[hvr]

Consider the following pathological autogen.cabal case:

cabal-version:       2.0
name:                autogen
version:             0
build-type:          Simple

executable autogen
  main-is:             Main.hs
  build-depends:       base
  default-language:    Haskell2010

This will fail to build due to filesystem name clashes

[quasicomputational]

Related: #4085

[hvr]

@quasicomputational not really :-)

library pkgname is in fact invalid

executable autogen should work

iow, totally different issues