Merge lgCanon and lgInstance

We previously recorded the canonical package and the instance of a link group
separately, but it turns out we can always record these together: when we know
one we know the other. This makes various choices a lot clearer.

Addresses https://github.com/haskell/cabal/pull/2500#commitcomment-10798823
Addresses https://github.com/haskell/cabal/pull/2500#commitcomment-10798983
Addresses https://github.com/haskell/cabal/pull/2500#commitcomment-10799096
Addresses https://github.com/haskell/cabal/pull/2500#commitcomment-10799127
Addresses https://github.com/haskell/cabal/pull/2500#commitcomment-10799454
Addresses https://github.com/haskell/cabal/pull/2500#commitcomment-10799467
Addresses https://github.com/haskell/cabal/pull/2500#commitcomment-10799561
Addresses https://github.com/haskell/cabal/pull/2500#commitcomment-10799579
Addresses https://github.com/haskell/cabal/pull/2500#commitcomment-10799827
Addresses https://github.com/haskell/cabal/pull/2500#commitcomment-10799865
Addresses https://github.com/haskell/cabal/pull/2500#commitcomment-10799896