[prototype runtime] fails due to `cannot load such file -- sorted_set` with Ruby 3.0

Hello, I find a bug that the rbs prototype runtime Object command fails on Ruby 3.0 (it succeeds on Ruby 2.7).
Please see the reproduction below for details.

Reproduction

Note: I'm using Docker to reduce any environment-specific problems.

Ruby 3.0

rbs prototype runtime Object fails with LoadError:

$ docker run --rm -it rubylang/ruby:3.0.0-focal bash
# gem install rbs:1.0.4
Fetching rbs-1.0.4.gem
Successfully installed rbs-1.0.4
1 gem installed

# rbs prototype runtime Object
/usr/local/lib/ruby/3.0.0/set/sorted_set.rb:4:in `rescue in <top (required)>': The `SortedSet` class has been extracted from the `set` library.You must use the `sorted_set` gem or other alternatives. (RuntimeError)
	from /usr/local/lib/ruby/3.0.0/set/sorted_set.rb:1:in `<top (required)>'
	from <internal:/usr/local/lib/ruby/3.0.0/rubygems/core_ext/kernel_require.rb>:85:in `require'
	from <internal:/usr/local/lib/ruby/3.0.0/rubygems/core_ext/kernel_require.rb>:85:in `require'
	from /usr/local/lib/ruby/gems/3.0.0/gems/rbs-1.0.4/lib/rbs/prototype/runtime.rb:305:in `const_get'
	from /usr/local/lib/ruby/gems/3.0.0/gems/rbs-1.0.4/lib/rbs/prototype/runtime.rb:305:in `block in generate_constants'
	from /usr/local/lib/ruby/gems/3.0.0/gems/rbs-1.0.4/lib/rbs/prototype/runtime.rb:304:in `each'
	from /usr/local/lib/ruby/gems/3.0.0/gems/rbs-1.0.4/lib/rbs/prototype/runtime.rb:304:in `generate_constants'
	from /usr/local/lib/ruby/gems/3.0.0/gems/rbs-1.0.4/lib/rbs/prototype/runtime.rb:385:in `generate_class'
	from /usr/local/lib/ruby/gems/3.0.0/gems/rbs-1.0.4/lib/rbs/prototype/runtime.rb:45:in `block in decls'
	from /usr/local/lib/ruby/gems/3.0.0/gems/rbs-1.0.4/lib/rbs/prototype/runtime.rb:42:in `each'
	from /usr/local/lib/ruby/gems/3.0.0/gems/rbs-1.0.4/lib/rbs/prototype/runtime.rb:42:in `decls'
	from /usr/local/lib/ruby/gems/3.0.0/gems/rbs-1.0.4/lib/rbs/cli.rb:597:in `run_prototype'
	from /usr/local/lib/ruby/gems/3.0.0/gems/rbs-1.0.4/lib/rbs/cli.rb:113:in `run'
	from /usr/local/lib/ruby/gems/3.0.0/gems/rbs-1.0.4/exe/rbs:7:in `<top (required)>'
	from /usr/local/bin/rbs:23:in `load'
	from /usr/local/bin/rbs:23:in `<main>'
<internal:/usr/local/lib/ruby/3.0.0/rubygems/core_ext/kernel_require.rb>:85:in `require': cannot load such file -- sorted_set (LoadError)
	from <internal:/usr/local/lib/ruby/3.0.0/rubygems/core_ext/kernel_require.rb>:85:in `require'
	from /usr/local/lib/ruby/3.0.0/set/sorted_set.rb:2:in `<top (required)>'
	from <internal:/usr/local/lib/ruby/3.0.0/rubygems/core_ext/kernel_require.rb>:85:in `require'
	from <internal:/usr/local/lib/ruby/3.0.0/rubygems/core_ext/kernel_require.rb>:85:in `require'
	from /usr/local/lib/ruby/gems/3.0.0/gems/rbs-1.0.4/lib/rbs/prototype/runtime.rb:305:in `const_get'
	from /usr/local/lib/ruby/gems/3.0.0/gems/rbs-1.0.4/lib/rbs/prototype/runtime.rb:305:in `block in generate_constants'
	from /usr/local/lib/ruby/gems/3.0.0/gems/rbs-1.0.4/lib/rbs/prototype/runtime.rb:304:in `each'
	from /usr/local/lib/ruby/gems/3.0.0/gems/rbs-1.0.4/lib/rbs/prototype/runtime.rb:304:in `generate_constants'
	from /usr/local/lib/ruby/gems/3.0.0/gems/rbs-1.0.4/lib/rbs/prototype/runtime.rb:385:in `generate_class'
	from /usr/local/lib/ruby/gems/3.0.0/gems/rbs-1.0.4/lib/rbs/prototype/runtime.rb:45:in `block in decls'
	from /usr/local/lib/ruby/gems/3.0.0/gems/rbs-1.0.4/lib/rbs/prototype/runtime.rb:42:in `each'
	from /usr/local/lib/ruby/gems/3.0.0/gems/rbs-1.0.4/lib/rbs/prototype/runtime.rb:42:in `decls'
	from /usr/local/lib/ruby/gems/3.0.0/gems/rbs-1.0.4/lib/rbs/cli.rb:597:in `run_prototype'
	from /usr/local/lib/ruby/gems/3.0.0/gems/rbs-1.0.4/lib/rbs/cli.rb:113:in `run'
	from /usr/local/lib/ruby/gems/3.0.0/gems/rbs-1.0.4/exe/rbs:7:in `<top (required)>'
	from /usr/local/bin/rbs:23:in `load'
	from /usr/local/bin/rbs:23:in `<main>'

Ruby 2.7

rbs prototype runtime Object has no problems:

$ docker run --rm -it rubylang/ruby:2.7.2-bionic bash
# gem install rbs:1.0.4
Fetching rbs-1.0.4.gem
Successfully installed rbs-1.0.4
1 gem installed

# rbs prototype runtime Object
class Object < BasicObject
  include PP::ObjectMixin

  include JSON::Ext::Generator::GeneratorMethods::Object

  include Kernel
end

Object::ARGF: ARGF.class

Object::ARGV: Array[untyped]

Object::CROSS_COMPILING: untyped?

Object::ENV: Object

Object::FALSE: bool

Object::NIL: untyped?

Object::RUBY_COPYRIGHT: String

Object::RUBY_DESCRIPTION: String

Object::RUBY_ENGINE: String

Object::RUBY_ENGINE_VERSION: String

Object::RUBY_PATCHLEVEL: Integer

Object::RUBY_PLATFORM: String

Object::RUBY_RELEASE_DATE: String

Object::RUBY_REVISION: String

Object::RUBY_VERSION: String

Object::STDERR: IO

Object::STDIN: IO

Object::STDOUT: IO

Object::TOPLEVEL_BINDING: Binding

Object::TRUE: bool

Environment

• Ruby: ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-linux]
• RBS: rbs 1.0.4

Start at lib/rbs/prototype/runtime.rb around line 305, where the reported Ruby 3.0 stack trace shows constant loading reaches sorted_set. Reproduce with rbs prototype runtime Object on Ruby 3.0 and compare with Ruby 2.7; done means the command completes successfully on Ruby 3.0 without the reported LoadError.