This commit is a number of refactorings that I needed to do
while fixing bugs with internal libraries support.

- With internal libraries, it becomes especially clear that
  cabal_macros.h and Paths_foo.hs need to be done per-component.
  It is done!

  This change breaks BC in an important way: the preprocessor
  interface now takes a ComponentLocalBuildInfo along with the
  BuildInfo and LocalBuildInfo.  This means that if you implemented
  a custom preprocessor, or called 'preprocessComponent' in a custom
  Setup, you will have to make sure you pass the right
  ComponentLocalBuildInfo.  Some sub-notes:

    - While I was mucking about cabal_macros.h, I updated it to have
      two new macros: CURRENT_COMPONENT_ID (an alias for
      CURRENT_PACKAGE_KEY, but using modern terminology) and
      LOCAL_COMPONENT_ID (which refers to the public library; we use
      this in Cabal's test suite but it's unclear what the general
      utility of this is.  See the TODO.)

    - checkForeignDeps has a hack where we hardcode the
      cabal_macros.h of the main library.  If we did the foreign dep
      check for every component individually that would be better,
      but I didn't want to roll it into this patch.

- The other piece I needed for internal libraries was per-component
  install directories; otherwise, internal libraries clobber each
  other.  absoluteInstallDirs now takes a ComponentId, which is used
  to determine what '$libname' expands to.  Generally, InstallPaths
  must be computed per component, c.f. #2836.  We're not TRULY
  per-component install paths, since some files are installed for
  the "per-package" InstallPaths (the one we computed for the
  library as a whole), but for libraries we have to compute
  InstallPaths for each one.

    - While doing this, ComponentLocalBuildInfo grew a new
      'componentId' field for non-library things.  This lets us
      treat InstallPaths expansion uniformly.

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