Introduce Distribution.Compat.Lens - #4701

This patch introduces Distribution.Compat.Lens, a small portion of lens. The need is two fold:

• to provide lenses to users of Cabal
• to use it internally in places where lenses shine (subjectively, see Check.hs)

The design is simple:

• Distribution.Compat.Lens have some definitions from lens.
• type modules have sibling .Lens module which exports the type and lenses, with the same name as fields. Users can either

import Distribution.Types.PackageDescription
import qualified Distribution.Types.PackageDescription.Lens as L

or

import Distribution.Types.PackageDescription.Lens

Latter variant is not yet viable, as not everything is lensified. But we can make progress there quite easily.

To better compare lens and "old-school" Haskell approaches see e.g. (I realized later, these are essentially same checks):

• lens checkUnusedFlags https://github.com/phadej/cabal/blob/2dbd57a4aba07828c9520c7692c63138cfcda658/Cabal/Distribution/PackageDescription/Check.hs#L1620-L1642
• vanilla checkConditionals https://github.com/phadej/cabal/blob/2dbd57a4aba07828c9520c7692c63138cfcda658/Cabal/Distribution/PackageDescription/Check.hs#L1561-L1597

This commit includes some lens defintions to show up, how the Cabal-lens
framework could look like.

As an example checks which used ad-hoc lenses use newly introduced
lens-framework.

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• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.

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