Bootstrap gem symbols from Rubydex graph

[Morriar]

Summary

Extracted from #2524. Instead of shelling out to Sorbet to get the symbol table for a gem's files, we use the Rubydex graph which is already available and faster. The engine symbols path also switches to Rubydex for consistency.

• Replace Sorbet's symbol table (symbols_from_paths) with Rubydex graph for bootstrapping gem symbols in the pipeline
• The gem graph is now always built (not conditional on include_doc), and reused for both symbol bootstrapping and documentation
• Remove gem_symbols from SymbolLoader and load_bootstrap_symbols from Pipeline since Rubydex handles both cases

[vinistock]

It seems we're separating 3 collections just to immediately merge them back. How about something like this?

Suggested change

	namespaces = graph.declarations.grep(Rubydex::Namespace).map(&:name)
	constants = graph.declarations.grep(Rubydex::Constant).map(&:name)
	aliases = graph.declarations.grep(Rubydex::ConstantAlias).map(&:name)
	(namespaces + constants + aliases).to_set
	graph.declarations.select do |decl|
	decl.is_a?(Rubydex::Namespace) ||
	decl.is_a?(Rubydex::Constant) ||
	decl.is_a?(Rubydex::ConstantAlias)
	end
	end.to_set