RFC: library visibility enforcement

[fgaz]

So, before 3.0 we want a way to enforce library visibility, but to do that we need an extra field in IPI, which would require a GHC release and would restrict multilibs to GHC >=8.8.

Instead, what about:

• enforcing visibility at the Cabal-the-lib level on GHC >=8.8 only
• on GHC <8.8 just enforcing it at the cabal-install level
• hoping that this doesn't happen

Update: we decided not to take the risk. Multilibs will only be supported from GHC 8.8

[fgaz]

#5848 does it. It also adds an override so cabal-install can add backward compatibility.