Add support for optional-packages

[RyanGlScott]

I can't use haskell-ci in the gl repo at the moment because haskell-ci won't generate the necessary commands to build the gl subdirectory, which is listed as optional-packages: gl in the cabal.project file.

[phadej]

Why gl is optional-package?

[RyanGlScott]

No idea. I don't make the news, I just report it.

[phadej]

https://cabal.readthedocs.io/en/latest/nix-local-build.html#cfg-field-optional-packages

Like packages, specifies a list of package locations containing local packages to be built. Unlike packages, if we glob for a package, it is permissible for the glob to match against zero packages. The intended use-case for optional-packages is to make it so that vendored packages can be automatically picked up if they are placed in a subdirectory, but not error if there aren’t any.

It seems, one should use packages, as gl.cabal is always there, isn't it? Even it's regenerated, it's still there (the old one).

[ekmett]

I intend to move to generating the gl.cabal file as well. It really should be generated now but I was being lazy.