Fixes #711. (Hopefully)

during --help.

Fixes #535.

Fixes #866.

Fixes #1183.

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.

Since this file is not intended to be edited by hand, don't print the default
values for empty fields in comments (just like cabal-dev).

Remove now-unused runTests.sh

Broken by 481c4214 back in 2008, and no
longer necessary.

Removing unused stuff in Distribution.Compat.Env

Add simple heuristic to cabal init to guess common extra-source-files

Drop code to handle unsupported compilers

This means tweaking CPP conditionals and file pragmas to remove handling
of compilers other than GHC, and GHC versions before LANGUAGE pragmas
existed.

Two more Haddock module attribute syntax updates

cf. revision 47cd57c3

Haddock module attributes specified with old syntax

This re-hides several modules that had been inadvertently unhidden by
the change in attribute syntax in Haddock 2.0.0.0 (nearly five years
ago!)

Make the UserConstraint parser non-ambiguous. 

`+++` and `<++` are right-associative, so `a +++ b +++ c <++ d` is parsed as
`(a +++ (b +++ (c <++ d)))`.

Fixes #1159.

s/AmbigousParse/AmbiguousParse/

Improve warning for old versions of HPC