Remove lib/pathname.rb and rb_provide("pathname")

All methods previously defined in lib/pathname.rb have been moved to
pathname_builtin.rb. Remove the now-empty lib/pathname.rb and add
rb_provide("pathname") so that require "pathname" succeeds as a no-op.

Also move the author and copyright header from the deleted file into
pathname_builtin.rb.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hsbt

lib/pathname.rb

@@ -120,6 +120,7 @@ InitVM_pathname(void)
     rb_define_method(rb_cPathname, "sub", path_sub, -1);
 
     rb_provide("pathname.so");
+    rb_provide("pathname");
 }
 
 void

pathname.c

@@ -1,5 +1,14 @@
 # frozen_string_literal: true
 #
+# = pathname.rb
+#
+# Object-Oriented Pathname Class
+#
+# Author:: Tanaka Akira <akr@m17n.org>
+# Documentation:: Author and Gavin Sinclair
+#
+# For documentation, see class Pathname.
+#
 # A \Pathname object contains a string directory path or filepath;
 # it does not represent a corresponding actual file or directory
 # -- which in fact may or may not exist.