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For now we don't build the Cabal userguide, but we should add markdown
support so that we can do so.

This is a follow-on to Simon's patch yesterday, developed
with him.  It cleans up the computation of how packages
are installed, and installs the right ones.

DPH is now using the public vector package instead of its internal
version.

vector and primitive are not "boot" packages; they aren't required to
build GHC, but they are required to validate (because we include DPH
when validating).

If you say './darcs-all get --no-dph' then you don't get DPH, vector,
or primitive.

Instead of the ghc-stage and ghc-stage2-package files in a package, we
now have a list of these in ghc.mk.  There are other similar lists (of
boot-packages and non-installable packages), so this is not too bad,
and is simpler.

While poking around in the top-level ghc.mk file I spotted various
opportunities to clean up and re-order some of the cruft that has
accumulated over time.

saying

  make TRACE=1

prints most of the macro calls and their arguments.  It's easy to
trace new macros; see rules/trace.mk.

rather than being repeated in the build system

It's not just when cleaning!

Stops the docs going out of sync with the code.

We need to include the DPH cleaning rules, even though we don't build DPH
when GhcProfiled = YES.

Reason: DPH uses Template Haskell and TH doesn't work 
in a profiled compiler

Revert this patch:
    Matthias Kilian <kili@outback.escape.de>**20090920181319
    Don't build haddock if HADDOC_DOCS = NO, and disable HADDOC_DOCS
        if GhcWithInterpreter = NO
    Haddock uses TcRnDriver.tcRnGetInfo, which is only available if
    GHCI is built. Set HADDOC_DOCS to NO if GhcWithInterpreter is NO,
    and disable the haddock build if HADDOC_DOCS = NO.

This fixes a problem with commands like gzip, where if $GZIP is exported
in the environment, then when make runs a command it'll put the Makefile
variable's value in the environment. But gzip treats $GZIP as arguments
for itself, so when we run gzip it thinks we're giving it "gzip" as an
argument.

Spotted by Marco Túlio Gontijo e Silva

When using the bootstrapping compiler, we now explicitly constrain
the version of boot packages (Cabal, extensible-exceptions, etc) to the
in-tree version, so that the build system is less fragile should the
user have a newer version installed for the bootstrapping compiler.