Use “Cannot find the declaration of method” for methods defined in Ruby but not in RBS

[ParadoxV5]

This helps with using Steep as an LSP by differentiating no-RBS methods from actually nonexistent methods.

---

ruby.rb:

class A
  # control
  module B
  end
  class C
  end
  
  # subjects
  def instance_meth
    0
  end
  new.instance_meth
  def self.class_meth
    0
  end
  class_meth

  # control
  nonexistent_method
end

rbs.rbs:

class A
end

steep check:

# Type checking files:

...................................................................................F

ruby.rb:3:9: [warning] Cannot find the declaration of module: `B`
│ Diagnostic ID: Ruby::UnknownConstant
│
└   module B
           ~

ruby.rb:5:8: [warning] Cannot find the declaration of class: `C`
│ Diagnostic ID: Ruby::UnknownConstant
│
└   class C
          ~

ruby.rb:12:6: [error] Type `::A` does not have method `instance_meth`
│ Diagnostic ID: Ruby::NoMethod
│
└   new.instance_meth
        ~~~~~~~~~~~~~

ruby.rb:16:2: [error] Type `singleton(::A)` does not have method `class_meth`
│ Diagnostic ID: Ruby::NoMethod
│
└   class_meth
    ~~~~~~~~~~

ruby.rb:19:2: [error] Type `singleton(::A)` does not have method `nonexistent_method`
│ Diagnostic ID: Ruby::NoMethod
│
└   nonexistent_method
    ~~~~~~~~~~~~~~~~~~

Detected 5 problems from 1 file

Preferred:

# Type checking files:

...................................................................................F

ruby.rb:3:9: [warning] Cannot find the declaration of module: `B`
│ Diagnostic ID: Ruby::UnknownConstant
│
└   module B
           ~

ruby.rb:5:8: [warning] Cannot find the declaration of class: `C`
│ Diagnostic ID: Ruby::UnknownConstant
│
└   class C
          ~

ruby.rb:9:6: [warning] Cannot find the declaration of method: `instance_meth`
│ Diagnostic ID: Ruby::UnknownMethod
│
└   def instance_meth
        ~~~~~~~~~~~~~

ruby.rb:13:6: [warning] Cannot find the declaration of method: `self.class_meth`
│ Diagnostic ID: Ruby::UnknownMethod
│
└   def self.class_meth
        ~~~~~~~~~~~~~~~

ruby.rb:19:2: [error] Type `singleton(::A)` does not have method `nonexistent_method`
│ Diagnostic ID: Ruby::NoMethod
│
└   nonexistent_method
    ~~~~~~~~~~~~~~~~~~

Detected 5 problems from 1 file