Make sure to pass the package key to ghc

In https://github.com/haskell/cabal/pull/2439 the invocation of Haddock is
changed to use Haddock's new `--package-name` and `--package-version` flags,
necessary for GHC >= 7.10 (Cabal issue
https://github.com/haskell/cabal/issues/2297 / Haddock issue
https://github.com/haskell/haddock/issues/353). However, this commit also
removes the `-package-name` argument to GHC. This is incorrect, as it means
that GHC will end up calling the package `main` and we end up with Haddock link
environments such as

    (trans_H9c1w14lEUN3zdWCTsn8jG:Control.Monad.Trans.Error.strMsg, main:Control.Monad.Error.Class)

Note that before this commit we ended up with

    (trans_H9c1w14lEUN3zdWCTsn8jG:Control.Monad.Trans.Error.strMsg, mtl-2.1.1:Control.Monad.Error.Class)

which is equally wrong as it uses a package source ID rather than a package key
(Haddock issue https://github.com/haskell/haddock/issues/362). Instead, we need
to pass _both_ `--package-name` and `--package-version` to Haddock, and
`-package-name` or `-this-package-key` to GHC, depending on the version.
Thankfully the infrastructure for chosing between `-package-name` and
`-this-package-key` is already in place, so we just have to make sure to
populate the `ghcPackageKey` field. After this commit the link environment
looks like

    (trans_H9c1w14lEUN3zdWCTsn8jG:Control.Monad.Trans.Error.strMsg, mtl_Koly6qxRZLf86guywd4tkE:Control.Monad.Error.Class)

which is correct.

Fixes #2309, #1889.

Fixes #2461.

Say 16 days, rather than 16.3 days. The precision is quite unnecessary.

Instead of the old http://hackage.haskell.org/packages/archive
we can just use http://hackage.haskell.org/ and avoid doing
an http redirect. The old one continues to work of course.

Put full package name and version in library names, fixes #2437.

Reconfigure when there is no cabal file, fix #1905

This reverts commit 7da2a8eb.

[ci skip]

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Added checks to see if files and directories exist

Auto-generate ChangeLog.md in cabal init

Treat -O2 in ghc-options as a cabal check "warning"

Cabal : Fix incomplete pattern matches.

cabal-install.cabal : Add missing test dependency.

Closes: https://github.com/haskell/cabal/issues/2447