Want to make use of chreekat's recent refactorings+additions in Init/Heuristics.hs.

This reduces the runtime of "cabal install --dry-run" on a large cabal
file (550 dependencies) from ~12s to 2.1s. Equivalency with previous
implementation verified with QuickCheck in ghci.

This is a guess: The speedup is mostly due to how to the previous
entries are added to the result of the recursive call. Previously, for
every entry to be added, the whole result list is mapped over (lots of
allocations), and then each tuple is taken apart and reconstructed with
a changed right value (again, lots of allocations). The new code
assembles a function in an accumulator that represents the required
update and applies it where needed, hence building the list and tuples
only once.

RFC: Adding git prefs to 'cabal init' author-lookup heuristics

Only remaining thing is to actually parse LANGUAGE pragmas in source files.

Add extra-html-files, for installing extra html files

Fixes #711. (Hopefully)

during --help.

Fixes #535.

Fixes #866.

Fixes #1183.

For example, you might have an images/ directory in your project, with
images that you refer to from Haddock with:

    <<images/foo.png>>

Then in your Cabal file you would include:

    extra-html-files: images/*.png

And these would both be packaged by sdist, and "cabal haddock" will
install them in:

    ~/.cabal/share/doc/PROJECT/html/images/*.png

Fixes #1167

Remove DefaultSetup.hs

My investigation suggests it became unnecessary for the build with
revision 21dc5578 (named
DefaultSetup.lhs at the time) but has hung around ever since.

Replace the 'unpack' command with a more general 'get'.

'cabal get PACKAGE' is the new name of 'cabal unpack'.

'cabal get --source-repository' reads the source-repositories from a package's
description, determines which VCS to use, and then creates a local repository or
branch of the package's repository.

'cabal get --source-repository=[head|this|...]' additionally allows to specify
which source-repository to use.

Based on the code originally written by John Millikin <jmillikin@gmail.com>.

I forgot to enable -threaded for the cabal-install exe when I split
cabal-install into an exe and a library part. Thanks to Tuncer Ayaz for the
heads-up.

Generalise 'ppField' and friends and remove customised copies from
D.C.PackageEnvironment.