Use hackage-security library to extract `.cabal` files from the index

[edsko]

• Use library to extract .cabal files (and verify signatures once we introduce author signing; until that time this is just an optimization).
• Avoid creation of the cabal-specific tar index in favour of the index created by hackage-security (though note that the latter does not contain any information about preferred-versions). This should speed up cabal update.
• Once cabal-install only access the index through the security library, rename 00-index.tar and co to 01-index.tar (unless we can about older cabal's access?)

[edsko]

We now use the hackage-security cache to construct the cabal-install cache, but everything else on this ticket will be delegated to phase 2.