Prefer require_relative for loading internal code

So that we don't need to search the LOAD_PATH, given that we know which
files we want to load in advance.

lib/uri.rb

@@ -92,13 +92,13 @@
 module URI
 end
 
-require 'uri/version'
-require 'uri/common'
-require 'uri/generic'
-require 'uri/file'
-require 'uri/ftp'
-require 'uri/http'
-require 'uri/https'
-require 'uri/ldap'
-require 'uri/ldaps'
-require 'uri/mailto'
+require_relative 'uri/version'
+require_relative 'uri/common'
+require_relative 'uri/generic'
+require_relative 'uri/file'
+require_relative 'uri/ftp'
+require_relative 'uri/http'
+require_relative 'uri/https'
+require_relative 'uri/ldap'
+require_relative 'uri/ldaps'
+require_relative 'uri/mailto'