More reliable check_spec'ing for Cover-compiled modules.

check_spec indirectly used code:which/0 to load the BEAM of a targeted
module.   If the module was compiled with {cover_enabled, true}, proper
failed to load the BEAM and resorted to recompiling the module with a
hardcode list of compilation options.  If the targeted module depended
on any additional flags (such as include dirs), the compilation failed
and check_spec returned an error.

This fix uses code:get_object_code/1 (instead of code:which/1) to
retrieve the BEAM for a given module, sidestepping the need to
recompile any Cover-compiled modules.