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]

Do we still need the include_doc flag? If memory serves, we added that because including docs was super slow, but with Rubydex the problem no longer exists.

Maybe we can clean up that flag