Also check duplicated mixins in shims and TODO files

[Morriar]

Motivation

While we check scopes and methods duplicated between generated RBIs and shims, we do not check anything about mixins.

Consider this:

# in generated sorbet/rbi/gems/foo@1.0.0.rbi

module Bar; end

class Foo
  include Bar
end

# in manually written shim sorbet/rbi/shims/gems/foo.rbi

class Foo
  include Bar
end

The shim file is actually useless and could be deleted without impacting type checking since the include is already known.

Implementation

We can reuse the indexing mechanism already in place for other properties to check for duplicates. We just need to look for RBI::Mixin nodes and RBI::RequiresAncestor nodes.

One limitation of the static approach is that we do not resolve constants when checking for duplicates so something like this would not be considered a duplicate:

module A
  class B
    include C
  end
end

module A
  class B
    include ::C
  end
end

Because we can't really ensure that C is the same thing as ::C at this level.

Tests

See automated tests.

Using this feature I tried this in core and found one file that can be safely removed.