Currently settings `documentation: true` enables documentation
generation via haddock for your whole package, including tests and benchmarks.
However, there are additional flags to control generation of
documentation for this "second class" documentation targets, which are
currently not honored at the cabal-install side of things. Namely,
`tests`, `benchmarks`, `executables`, etc. provided under the
`haddock` section in your `$CABAL_HOME/config`.

This patch adds a more sensible approach to documentation generation
via haddock. Also enabling `new-haddock` to generate documentation for
single components instead whole packages.

The behaviour works like this:

    - Setting `documentation: true` or passing
      `--enable-documentation` to cabal-install enable documentation
      for any component in the build plan honoring the respective
      flags for tests, benchmarks, exes, foreignlibs, etc.

    - Invoking new-haddock with a target selector will make sure
      the respective flags for "second class" doc targets are set
      correctly. E.g.

      $ new-haddock tests

      Will generate documentation for the testsuite of your package
      event if you have `tests: false` in your haddock section.

Refactoring removing `data ReadPackageIndexMode`

Enforce the backjump limit before creating the solver log. 

This is anticipated refactoring (since `readCacheStrict` will
be removed anyway once we remove the `cabal sandbox` code) is
motivated by workarounding a GHC 8.4.1 caused regression
(see https://ghc.haskell.org/trac/ghc/ticket/13930 for details)

Addresses #5201

Fix powershell transport

Look for transitive setup dependency on Cabal when choosing Cabal spec version.

Update regression test for issue #4154 after the fix for issue #415.

Forward ports

Add more lenses for PackageDescription and GenericPackageDescription

Fix new-install symlink location

Before there wasn't much feedback. Most times the only thing that got
printed was "Up to date".

It is set to the same directory used by the old cabal install command,
ie `~/.cabal/bin` on linux, as it will be used to "install" (symlink)
executables by new-install.

This commit addresses the comments in PR #5183.

cross compile exe extension

This ensures that we get the proper `hostPlatform` where possible, and only fall
back to the legacy `buildPlatform` in other cases.

This uses GHC 7.4 as a reference; i.e. we consider
the versions of boot libraries of GHC 7.4.[12] as the
lower bound we support.

Specifically, w/ GHC 7.4.1, these are the direct dependencies
the cabal solver will come up with (when also forcing
`mtl` to its lower bound):

  array-0.4.0.0
  base-4.5.0.0
  binary-0.5.1.0
  bytestring-0.9.2.1
  containers-0.4.2.1
  deepseq-1.3.0.0
  directory-1.1.0.2
  filepath-1.3.0.0
  ghc-prim-0.2.0.0
  mtl-2.1
  old-time-1.1.0.0
  parsec-3.1.13.0
  pretty-1.1.1.0
  process-1.1.0.1
  text-1.2.3.0
  time-1.4
  transformers-0.3.0.0
  unix-2.5.1.0

(cherry picked from commit 2b73e706)

(cherry picked from commit 80f3e714)