Make dead code indexing use a Model to index methods

[Morriar]

Follow-up on #564.

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

• Remove methods indexing from Deadcode::Indexer (8464dc2)
• Make the Deadcode::Plugins take Model::SymbolDef when calling on_define_method (7344efe)

There is 2 slight behavior changes

1. Before we were able to look at the nesting context when defining methods inside a no_commands block in Thor classes. So with this code:

   class MyCommand < Thor
     no_commands do
       def foo; end
     end
   end

   We could tell that foo was defined inside a no_commands do ... end block and thus not ignore it as other Thor commands.

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

   This is a regression that I deemed acceptable. We can revisit this later.

2. The Model::Builder ignores methods defined on receivers other than self. So def FOO.foo and def foo.foo will be ignored and not marked as dead. The previous indexing was clanky anyway and this form is pretty rare. We can revisit this later.

This PR is easier to review commit by commit.