Prefer require_relative for internal requires

`require_relative` is preferred over `require` for files within the same
project because it uses paths relative to the current file, making code
more portable and less dependent on the load path.

This change updates internal requires to use `require_relative` for
consistency, performance, and improved portability.

Ref:
- ruby/psych#522
- ruby/logger#20
- ruby/rdoc#658
- rubocop/rubocop#8748

lib/premailer.rb

@@ -6,8 +6,8 @@
 require 'addressable/uri'
 require 'css_parser'
 
-require 'premailer/adapter'
-require 'premailer/adapter/rgb_to_hex'
-require 'premailer/html_to_plain_text'
-require 'premailer/premailer'
-require 'premailer/cached_rule_set'
+require_relative 'premailer/adapter'
+require_relative 'premailer/adapter/rgb_to_hex'
+require_relative 'premailer/html_to_plain_text'
+require_relative 'premailer/premailer'
+require_relative 'premailer/cached_rule_set'

lib/premailer/executor.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 require 'optparse'
-require 'premailer'
+
+require_relative '../premailer'
 
 # defaults
 options = {

lib/premailer/premailer.rb

@@ -30,7 +30,7 @@
 # puts premailer.to_inline_css
 # ```
 #
-require 'premailer/version'
+require_relative 'version'
 
 class Premailer
   include HtmlToPlainText