Don't set mainProgram when it matches pname

[SuperSandro2000]

stdenv.mkDerivation falls back to pname when meta.mainProgram is not set. cabal2nix should follow that and only set mainProgram when it differes from pname.

[sternenseemann]

stdenv.mkDerivation falls back to pname

Nix does, not stdenv.mkDerivation. If meta.mainProgram is not set, it is not set.

There is a difference between not knowing the main program where with nix run its reasonable to make an assumption, and other cases where it may not be reasonable to do so.

cabal2nix may even not emit a mainProgram binding if an executable of the same name exists, indicating that it was not able to determine for sure that that executable is the main one.

Consequently, I don't see a reason to change this.