Download artifact repositories

[edpittol]

This PR is a fix to #453.

The artifact repository does not have references such as VCS and Path. Currently shasum is considered a reference, but it is not necessary. For artifacts, the value of the reference property must be empty because an artifact does not have a reference to some point.

This code allows you to download VCS repositories and artifacts. But I believe the architecture can evolve so that each dist definition in the repository has its own specification to represent the composer specification.

I believe the Composer specification is just in the code. I didn't find any public reference documents about packages.json. The settings are at:

• Composer\Repository\ArtifactRepositor::getComposerInformation
• Composer\Repository\PathRepository::initialize
• Composer\Repository\VcsRepository::preProcess