FEAT: add Request class to track multiple jobs.
REF: tweak private var name in Repository + add test for find_package.
BUG: fix typo in Request.
ENH: parse_requirement_string now returns empty sets instead of None.
ENH: Version and None can now be compared.

This will be needed for traitlets use.
REF: rename Package to PackageInfo.

We renamed to PackageInfo to stress it only contains information
required by the dependency solver.
ENH: bundle traitlets.

I want to experiment with modeling the different classes as traits.
Traits is most likely overkill and we can't afford the C dependency
anyway.
REF: use traitlets for PackageInfo. Most solver's APIs are broken.
ENH: Repository class is now using traitlets.
ENH: repository is now a property of PackageInfo.
FEAT: add support for parsing '*' requirement.
ENH: Pool is now a traitlets class.
BUG: fix PackageInfo ctor to work wo call by name.
BUG: fix doctest for PackageInfo.
ENH: Request is now a traitlets-based class.
REF: rename clause->rule modules.
FEAT: add id_to_literal conversion to Pool.
ENH: PackageRule is now a traitlets.
FEAT: add RuleSet class.

This class may be superflous, but for now, we will just have a simple
version of it to mimic Composer design. Should be easy to scrap later
on.
BUG: update to new traitlets that did not make Set available.