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.

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

during --help.

Fixes #535.

Fixes #866.

Fixes #1183.

'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).

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

cf. revision 47cd57c3

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

Fixes #1159.

Makes it possible to write unit tests for code under Distribution.Client.*.

This will allow it to be set in config files too.

Implemented by creating an install plan for ["add-source-dep-1", ...,
"add-source-dep-N", "."], pruning "." from this plan and then doing all
remaining installs in the plan before building the current package. This way,
all reverse dependencies of add-source packages needed to install the current
package are also reinstalled.

Splits 'D.C.Install.install' into three parts:

    * makeInstallContext - load common data
    * makeInstallPlan    - produce the install plan
    * processInstallPlan - actually perform the installations

This allows to manipulate the install plan produced with 'makeInstallPlan'
before performing the installations with 'processInstallPlan'. The high-level
'install' action is still present; most clients should use it instead.