Cabal does not handle main-is in conditionals well

[andreasabel]

twee/twee.cabal

Lines 47 to 52 in eb4b83b

	if flag(parallel)
	main-is: ParallelMain.hs
	build-depends: async, unix
	c-sources: executable/link.c
	else
	main-is: Main.hs

See upstream issues:

• Cabal sdist concatenates names of conditional Main module names haskell/cabal#9724
• Regression on master: main-is not picked up from conditional haskell/cabal#9742