Minor fixes to the Tapioca DSL compiler

This commit fixes a couple of issues with the Tapioca DSL compiler:

1. We have to check for the existance of the `Measured::Rails` class and not `Measured::Rails::ActiveRecord` since the latter might not have been loaded yet by the time we do the check for the constant.
2. It is better and safer to use the qualified name of the field class type for using inside signatures, so that we have `::Measured::Length` instead of `Measured::Length`.

lib/tapioca/dsl/compilers/measured_rails.rb

@@ -1,7 +1,7 @@
 # typed: strict
 # frozen_string_literal: true
 
-return unless defined?(::Measured::Rails::ActiveRecord)
+return unless defined?(::Measured::Rails)
 
 module Tapioca
   module Dsl
@@ -88,16 +88,18 @@ def self.gather_constants
         sig { params(model: RBI::Scope).void }
         def populate_measured_methods(model)
           constant.measured_fields.each do |field, attrs|
-            klass = attrs[:class].to_s
+            class_name = qualified_name_of(attrs[:class])
+
+            next unless class_name
 
             model.create_method(
               field.to_s,
-              return_type: as_nilable_type(klass)
+              return_type: as_nilable_type(class_name)
             )
 
             model.create_method(
               "#{field}=",
-              parameters: [create_param("value", type: as_nilable_type(klass))],
+              parameters: [create_param("value", type: as_nilable_type(class_name))],
               return_type: "void"
             )
           end