Use Cabal directly in place of ghc-cabal + make build root configurable (#531)

* Use Cabal directly in place of ghc-cabal; make build root configurable.

This commit implements two significant changes (that were not easy to
separate):

- Don't use ghc-cabal anymore for getting information about Haskell packages.
  We now instead directly use Cabal-the-library.

- Make the build root configurable. This effectively gets rid of the inplace
  logic and allows us to place _all_ build artefacts in some directory of
  our choice, by passing '--build-root <some path>' to hadrian.

The code for this was mostly taken from #445.

* fix documentation rules

* remove some leftover unrelated, commented-out code

* more documentation fixes, address some feedback

* cleanup

* more cleanup

* boot and configure explicitly in travis CI scripts

* update cabal/ghc versions in .travis.yml (8.0.x not supported anymore)

* temporarily disable dynamic ways in Settings.Default

* update appveyor script

* travis: when booting with 8.2.2, build a complete stage2 compiler

* Fix CI?

Try to fix the CI by adding the `debug` rts way back in.

* Update Quickest.hs

Replicate the make build systems build flavours.

* Update .travis.yml

- Run selftest, and build in separate instances.
- try with python2
- and unify mac to stage2

* Update .travis.yml

upgrade python on mac

* [travis] os x: test the freshly built ghc

* Get rid of two unused GhcCabalMode constructors

* fix ghc-split rule, get rid of Install/Wrappers rules

* address more feedback

* ConfiguredCabal -> PackageData, more comments, more feedback addressed

* make the complete stage 2 build the default

* use a dummy package instead of base in Rules.hs

* update CI scripts

* attempt at fixing hadrian's -c option

* .travis.yml: use -c everywhere again

* travis: back to explicit './boot && ./configure'

* update README.md and doc/user-settings.md to reflect configurable build root

* some more feedback