Better constant typing

[soutaro]

Found and fixed two bugs.

1. The :const node didn't have type if it's unknown
2. Reporting all of the constant components look too noisey (see below)

Before:

lib/rbs_protobuf/translator/base.rb:17:22: [information] Cannot find the declaration of constant: `Google`
│ Diagnostic ID: Ruby::UnknownConstant
│
└         @response ||= Google::Protobuf::Compiler::CodeGeneratorResponse.new
                        ~~~~~~

lib/rbs_protobuf/translator/base.rb:17:30: [information] Cannot find the declaration of constant: `Protobuf`
│ Diagnostic ID: Ruby::UnknownConstant
│
└         @response ||= Google::Protobuf::Compiler::CodeGeneratorResponse.new
                                ~~~~~~~~

lib/rbs_protobuf/translator/base.rb:17:40: [information] Cannot find the declaration of constant: `Compiler`
│ Diagnostic ID: Ruby::UnknownConstant
│
└         @response ||= Google::Protobuf::Compiler::CodeGeneratorResponse.new
                                          ~~~~~~~~

lib/rbs_protobuf/translator/base.rb:17:50: [information] Cannot find the declaration of constant: `CodeGeneratorResponse`
│ Diagnostic ID: Ruby::UnknownConstant
│
└         @response ||= Google::Protobuf::Compiler::CodeGeneratorResponse.new
                                                    ~~~~~~~~~~~~~~~~~~~~~

After:

lib/rbs_protobuf/translator/base.rb:17:22: [information] Cannot find the declaration of constant: `Google`
│ Diagnostic ID: Ruby::UnknownConstant
│
└         @response ||= Google::Protobuf::Compiler::CodeGeneratorResponse.new
                        ~~~~~~

This would look better. 💪