add way to query package dependency graph

[ganesh]

I'd like to be able to list all the forward or reverse dependencies of a package, optionally recursively.

The attached patch makes a 'cabal depends' command that does this, but Duncan (quite reasonably) doesn't want a proliferation of top-level commands. Instead it should be integrated into cabal info in some appropriate way, but this probably requires some refactoring of that command first.

The attached patch could also be improved in (at least) two ways:

• Deal with package version numbers as well as names. This involves matching the constraints on version numbers against specific version numbers.
• Use a less naive transitive closure calculation.

[kosmikus]

I definitely want something like this.

[kosmikus]

Vaguely related to #889.