add link.py from ronda verbatim

will modify to work without ronda.Packages later
remove usage of ronda.Packages

behave as if there were no packages providing the library: just call out an external linkage
in case of external linkage: allow process_action_items to proceed rather than hit breakpoint
consolidate repeated "if is_build" conditional
consolidate repeated lookup creation functionality

and give a descriptive name to what its doing
Directy use handle_link_errors from link.py

Don't rely on config's dynamic loading
Remove dynamic loading of link_errors_handler and associated logic

no longer used by main_build.py
remove external depedency: continuum.debug.iset_trace

replace with pdb.set_trace, though needs to be removed before merge
remove unused import

also remove now unused Imports header
move import sys to top of file

and add back Imports header
cull unused branch in process_action_items

relies on a flag that will only be set when ronda.Packages functionality is integrated
modify debugger invocation

and add a nice print notification
Merge pull request #143 from dlovell/rpath-sdl

Merge rpath-sdl.