Add way to exclude generated modules from sdists

[lpotmann]

In one of my projects, I am machine-generating a number of modules, which other parts of the code then import. These need to be specified in the .cabal file, otherwise hard-to-debug linker errors happen. However, then they get included in sdist tarballs, or sdist chokes if they're not present, which is undesirable. This patch lets generated modules be excluded from sdists on the command line with a --generated-module option.

(An alternative (and perhaps more robust) approach would be to add a generated-modules field to cabal files, but that's only just occurred to me and this patch is sitting here!)

[23Skidoo]

Can't you use the same trick as @dcoutts described in #1557 (generate a .buildinfo and list your generated modules there)?

[tibbe]

I definitely think we should go with generated-modules. We ought to track these regardless as e.g. distributed builds won't work otherwise.

[tibbe]

@lpotmann I'd like to see generated-modules. Are you interested in working on that?

[ttuegel]

I'm going to close this because it's pretty clear that this belongs in a generated-modules field of the package description (it's static configuration data, not something that can change at runtime).