net/http: use require_relative to reduce syscalls

require_relative speeds up loading of files by reducing path
lookups.  On a clean install with RubyGems-enabled,
"ruby -rnet/http -e exit" shows a reduction in failed open(2)
syscalls from 410 to 350 (x86-64 GNU/Linux).

I could not measure a time difference on my Linux-based
machines, however this should be noticeable to users of other
kernels with worse syscall and VFS performance than Linux.

Further use of require_relative will reduce lookups in other
places.

* lib/net/http.rb: use require_relative
  [ruby-core:78285] [Feature #12973]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60388 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/net/http.rb

@@ -20,7 +20,7 @@
 # See Net::HTTP for an overview and examples.
 #
 
-require 'net/protocol'
+require_relative 'protocol'
 require 'uri'
 
 module Net   #:nodoc:
@@ -1628,17 +1628,17 @@ module Net   #:nodoc:
 
 end
 
-require 'net/http/exceptions'
+require_relative 'http/exceptions'
 
-require 'net/http/header'
+require_relative 'http/header'
 
-require 'net/http/generic_request'
-require 'net/http/request'
-require 'net/http/requests'
+require_relative 'http/generic_request'
+require_relative 'http/request'
+require_relative 'http/requests'
 
-require 'net/http/response'
-require 'net/http/responses'
+require_relative 'http/response'
+require_relative 'http/responses'
 
-require 'net/http/proxy_delta'
+require_relative 'http/proxy_delta'
 
-require 'net/http/backward'
+require_relative 'http/backward'