Add -w flag to cabal fetch

[nh2]

I can cabal fetch mypackage.cabal to fetch all dependencies declared in a cabal file.

However, cabal will only do that for my default ghc; the packages that have to be downloaded for a different ghc version might be different.

It would be great if there was a cabal fetch -w flag, like there is for e.g. cabal install.

[23Skidoo]

The same applies to flags, maybe also OS/arch. Instead of adding a -w flag, it'd be better to make cabal fetch download all packages required for any possible combination of flags/compiler/arch/OS.