No longer needed now that the world target is handled via UserTarget.

Also minor tweak to InstallPlan.remove

Helps to clarify things now that different kinds of packages
are fetched in different ways.

Allows shorter and clearer code for the various ways the resolver is used.

The new user target system requires a change in how cabal info works.
Instead of just giving package names and looking them up in the
available package index, we can now specify names with versions or
version constraints which means we want the info to be about that
version in particular. We now list many installed and available
versions and mark which ones are preferred or not. Also fix a bug
for packages that are only installed, not available.

This will allow us to increase the range of targets that cabal
commands can support. The new ones are local directory targets,
local cabal files, local tarballs and remote tarballs by URL.
Also a better way of doing the special "world" target.

Works for any package identified by PackageLocation
rather than just for repo packages.

So we can now use PackageLocation FilePath or Maybe FilePath to
describe what we know about the fetch status of package tarballs.

Currently just a renamed copy of UnresolvedDependency but called
WorldPkgInfo and defined in the World module. This is in preparation
to remove all other uses of the UnresolvedDependency type.

And remove import list for Types module, just import it all

And rename fetchPackage function to the more accurate fetchRepoTarball

We can just use "." instead of Nothing

Move the ambiguity checking to the only use site
Both normal and substring search now return [(PackageName, [pkg])]

Move it next to the --upgrade-dependencies flag

This flag installs only the dependencies of the packages that were
explicitly mentioned on the command line. This is useful for using
cabal-install in development environments, where the developer needs
the dependencies to build the package in development, but does not yet
want to install the package itself.

Sigh, having to use shell script and regexps...

Use "maybeDecompress" to handle broken proxies that transparenty decompress network streams. Closes #622, #686. Cabal update could fail in some cases, see http://trac.haskell.org/http/ticket/109283

Fixes #741. Patch contributed by Jan Braun <janbraun@gmx.net>

There's still something wrong with the recursive dir traversal.
It fails in some large cases.

cabal upgrade now gives an error message telling people to use install
or, if they know what they're doing, install --upgrade-dependencies

Allows fetching one or more packages but without fetching their
dependencies and thus not requiring that a consistent install plan
can be found. On the other hand --no-deps means that there is no
guarantee that the fetched packages can actually be installed.

Not yet connected up to the user interface.