Resolve #6013: Warn if packages and optional-packages are both empty

[phadej]

Now there's a warning

Warning: There are no packages or optional-packages in the project
cabal: There is no <pkgname>.cabal package file or cabal.project file. To
build packages locally you need at minimum a <pkgname>.cabal file. You can use
'cabal init' to create one.

For non-trivial projects you will also want a cabal.project file in the root
directory of your project. This file lists the packages in your project and
all other build configuration. See the Cabal user guide for full details.

It's subtle, but that's the issue with how we output warnings, not the warning itself.

I'm only making it a warning at this point, as I'm not 100% sure if that setup may still be useful for something. Playing safe for 3.2

[phadej]

Related: #6144

[phadej]

Backported in #6430