Make dead code indexing use a Model to index constants

[Morriar]

Follow-up on #563.

This time, we switch the indexing of constants to Model:

• Remove constants indexing from Deadcode::Indexer (41250bb)
• Make the Deadcode::Plugins take Model::SymbolDef when calling on_define_constant (7894f64)

There is a slight behavior change, before we were able to look at the nesting context when defining T::Enum constants. So with this code:

class Foo < T::Enum
  CONST = new
end

We could tell that CONST wasn't defined inside a enums do ... end block and thus ignore CONST within the sorbet plugin.

To avoid carrying the enums do ... end information and putting pressure on the GC, I dropped this feature, so CONST will be marked as ignored.

In practice this case doesn't happen much, this is a regression that I deemed acceptable.

This PR is easier to review commit by commit.