Remove default `optional-packages`, i.e .make it empty.

[phadej]

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

Which is just confusing. See e.g. phadej/cabal-fmt#6

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I never benefited from default optional-packages

[phadej]

@llelf please explain why you think this is 👎

[phadej]

Worse: if there exists cabal.project, then the optional-packages: is actually empty. So ./*/*.cabal is a value in implicit (non-existing) cabal.project.

In that sense, the documentation is misleading. (same applies to packages field too).

[gbaz]

Its pretty convenient to be able to vendor libs just by tossing them into the source tree, for hacking purposes. So I feel quite mixed on this.

[phadej]

@gbaz it's easy to add cabal.project as you know about this feature.

packages: .
optional-packages: vendor/*/*.cabal

However to figure out why things don't work due implicit optional-packages: **/*.cabal, when there are no cabal.project around had tripped me (and beginners) enough time so I think that this is a misfeature.

If you disagree, please amend the documentation so beginners will find that they will need to write trivial

packages: .

cabal.project if there are some stuff in subdirectories (namely *.cabal files - they don't need to be actual packages).

I think it's easier to explain the benefit of optional-packages then to how turn it off. Especially as latter have to be easily discoverable. (Solver errors don't give a hint, see original messages; these could be improved to tell where from additional constraints are coming, but that is separate issue).