Fix documentation for operating_system.rb path

The require looks in rubygems/defaults/operating_system.rb
Respect default_ext_dir_for in require_paths

Now Rubygems respects default_ext_dir_for which allows rubygems
packagers to move extensions to the directories they want.

Fixes #852
Add #path accessor for Source::SpecificFile

This improves debugging.
Add LocalSpecification#pretty_print
Improve DEBUG_RESOLVER output

Pretty-print output

Add list of possible entries

Output to $stderr as this is debugging info