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User installs by default elsewhere. Note that this default only
affects new config files. Existing windows users will need to
change their config file or delete it and let it be re-created.

The bogusResolver is used for compilers like hugs and nhc98 where
we do not know what packages are already installed. We do it by
altering the descriptions of the packages we're going to install
to remove all of their dependencies. Doing this gives us a valid
install plan. We rely on hope to minimise the difference between
this install plan and reality.

Takes account of the ProxyEnabled reg key
Allow proxy uris that lack the http:// uri schema.
This should really be moved into the HTTP package.

so it now actually works to say: $ cabal install foo -fbar

We don't actually have per-package flags yet. All -f flags
on the install/upgrade command line apply to every paclage.

Don't bother putting the server url into each report
since we do not want to upload that information anyway.

and add a function to parse a whole logfile of reports

This is so that we remember which server to send the reports to.
This is pretty important if you're using a private as well as a
public hackage server.

Instead of just using the latest version we use the best version
according to the following preferences in priority order:
- the same version as cabal-install was itself built with
- the same major version number as cabal-install was built with
- a stable version of Cabal (even second digit of major number)
- the latest version

The available package index is loaded lazily so if we can avoid
forcing all the packages then we can save a huge amount of slow text
parsing. So select out the maximal subset of the index that we could
ever need based on the names of the packages we want to install. For
the common case of installing just one or two packages this cuts
down the number of packages we look at by a couple orders of
magnitude. This does not help with the installed index which is read
strictly, though most people do not (yet) have hundreds of installed
packages, so that's less of an immediate problem.

Allows us to use cabal-install to build packages where we build the
Setup.hs script using an older version of the Cabal library.
This is important for packages that claim to need an older version
and it's also pretty useful for testing.

It is important to know which version of the Cabal lib the setup
script has been built with because it affects what flags we can pass
to it. So we now store the version in dist/setup/setup.version.
If the version number changes then we rebuild the setup binary.

Otherwise we end up saying packages failed because they depend
on themselves which failed to install, which is silly.

The reverse deps may have already failed due to depending on another
failed package. So do not assume all reverse deps are in the
configured state. If they are in the failed state then ignore them.

This is because a package can depend on several packages and if one
of the deps fail then it fails but it still depend on the others
that did not fail. This fixes an assertion failure in larger builds.

Although it is expensive, we want to catch problems early.