mass-scan: Create PurlDB index of all major package registries and Linux distributions

[pombredanne]

We need to populate a PurlDB index with all major package registries and Linux distributions. This implies not actually fetching and indexing all the code, but holding a list of all the packages. We should consider only the major ones:

• npm
• pypi
• maven
• nuget
• rubygems
• fedora
• debian
• alpine