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APISet now accepts a dependency API URI

This allows the choice of different APIS.
Allow SpecFetcher creation with alternate sources

This allows IndexSet to work with a subset of Gem.sources in case not
all sources support newer APIs.
Gem::IndexSet can now work with a single source

This allows different sources to use different APIs.
Add Source#dependency_resolver_set

This returns a Set which can be used to resolve gems for this source.
The upcoming BestSet will use this.
Set up index metadata files with spec_fetcher

Now util_setup_spec_fetcher does everything necessary for SpecFetcher to
work.
Add BestSet which picks the most advanced API

BestSet looks at each source and chooses the most advanced API for each
source.