Add AvailableSet#remote= to fix test failures

AvailableSet is given to the resolver but does not subclass
Resolver::Set nor live in the correct directory.  This should be
remedied at some point.
Always create @remote_set for InstallerSet

We can decide to use it later if we're doing remote fetches or not.
Do not flatten BestSet

Now that a BestSet is resolved lazily to avoid network requests
composing it won't work as it will be empty.
Propagate remote flag to RequestSet for resolution

This makes domain: :local work again for the dependency installer.