Add export-depends

[phadej]

Describe the feature request

I would like .cabal format to have export-depends. This field would act as build-depends, but these dependencies won't be passed to the compiler.

The motivation is to avoid false positives when using reexported-modules. The reexports are handled by Cabal itself, and as GHC doesn't see that usage, it may incorrectly report dependencies as unused. From GHC POV, it is correct, they are not true build dependencies.

Additionally it would be great if reexport-modules functionality reported unused export-depends to complement -Wunused-packages from GHC.

[phadej]

See also #2965 for plugin-depends feature request.