[WIP] Split gem sources in lockfile

[chalkos]

Tasklist

• remove the rubygems aggregate
• link every gem to a single remote, possibly by grouping multiple gems under a source and managing multiple sources
• allow one global source that is chosen when no other source is present
• by default, the global source is the local source
• throw an exception when a gem could have multiple sources (see case 1 below)
• change the lockfile format to accommodate the changes above

---

Case 1: When a gem may be obtained from multiple sources.

source "https://rubygems.org"
gem "rake"

source "https://sidekiq.io" do
  gem "sidekiq-pro"
end

sidekiq-pro depends on sidekiq, which is hosted on rubygems.org (and not hosted on sidekiq.io)
so the gem sidekiq has no source, and could be available from rubygems.org, sidekiq.io, or both

Solutions:

• if it’s only available from one source, we need to lock it to that one
• if it’s available from more than one, we need to throw an exception and tell the user that they have to tell us which source to use. Then the user edits the gemfile to something like:

source "https://rubygems.org"
gem "rake"
gem "sidekiq"

source "https://sidekiq.io" do
  gem "sidekiq-pro"
end

and this way all gems have a definite source.

[chalkos]

@indirect @segiddins I think that the task list covers everything we talked about. Feel free to comment/add anything else you can think of.

[segiddins]

@chalkos how's this coming along?

[segiddins]

Succeeded by #4714