Find useless shim definitions from payload

[Morriar]

When we clean shims we do not consider Sorbet's payload RBIs.

This means that when a definition in our shims becomes useless because it's been upstreamed, we never clean it.

We should use Sorbet's symbol table to bootstrap the index and remove the outdated definitions.

Problem: how to know if the symbol in the table is actually attached to a signature? Maybe we should add an option to Sorbet so it can dump its internal RBIs?

[Morriar]

Pushed the first part so we can extract the content of the payload from the Sorbet version used: sorbet/sorbet#5504.

[Morriar]

Done in #879 🎉