This is an omnibus patch, with the overall goal of making
LocalBuildInfo Great Again.  The essential ideas:

* New type 'TargetInfo' which bundles together 'ComponentLocalBuildInfo'
  and 'Component'.  Eventually, it will also record file paths / module
  targets.  This data structure is basically what you want; a lot of
  old Cabal code did lots of gyrations converting from
  'ComponentLocalBuildInfo' to 'Component' and vice versa, now
  it's all centralized.

* The "new" API for 'LocalBuildInfo' is in
  "Distribution.Types.LocalBuildInfo".  The general principle
  is, where we previous dealt in 'ComponentLocalBuildInfo',
  we now deal in 'TargetInfo'.  There are shockingly few
  functions we need!

* I've restored 'componentsConfigs' to its Cabal 1.24 signature
  for BC.

* I killed a number of unused functions from "Distribution.Simple.LocalBuildInfo":
  'getLocalComponent', 'maybeGetDefaultLibraryLocalBuildInfo',
  'maybeGetComponentLocalBuildInfo', 'checkComponentsCyclic' and
  'enabledComponents'.  For each I checked on Hackage that they were
  not used.

* 'getComponentLocalBuildInfo', 'withComponentsInBuildOrder' and
  'componentsInBuildOrder' are deprecated to encourage people
  to instead use the 'TargetInfo's to finger which components
  they want built.

* 'ComponentLocalBuildInfo' now stores internally the computed
  'componentInternalDeps', so that 'LocalBuildInfo' can simply store
  a graph of 'ComponentLocalBuildInfo'.

* The code in Configure has been streamlined to use our new Graph
  data type to great success.

* The type of 'runTest' changed to take a 'ComponentLocalBuildInfo',
  bringing it more in line with everything else.

* New function 'readTargetInfos' which combines 'readBuildTargets'
  and 'checkBuildTargets', which is what you really wanted anyway.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>