presentation of dependencies in the web interface

[Ross Paterson]

Dependencies between packages are obviously more complex now that we
have configurations.

The web interface now has an experimental presentation of these
dependencies transformed into disjunctive normal form, with the atoms
being simple version ranges.  It lacks tests of os, arch and impl,
which will need to be added later.  Still, people using configurations
without those tests might like to check what it does to their packages.