Make sure files from erl_opts->src_dirs are copied to .eunit for cover
inspection.

Without this patch cover failed to find source files which were in
src_dirs, and threw the following exception:

ERROR: eunit failed while processing
/path/to/project:
{'EXIT',{{badmatch,{error,no_source_code_found}},
         [{rebar_eunit,'-cover_analyze/3-fun-0-',1,
                       [{file,"src/rebar_eunit.erl"},{line,280}]},
          {lists,foreach,2,[{file,"lists.erl"},{line,1262}]},
          {rebar_eunit,cover_analyze,3,
                       [{file,"src/rebar_eunit.erl"},{line,279}]},
          {rebar_eunit,eunit,2,[{file,"src/rebar_eunit.erl"},{line,140}]},
          {rebar_core,run_modules,4,[{file,"src/rebar_core.erl"},{line,425}]},
          {rebar_core,execute,5,[{file,"src/rebar_core.erl"},{line,359}]},
          {rebar_core,process_dir0,6,[{file,"src/rebar_core.erl"},{line,223}]},
          {rebar_core,process_dir,4,
                      [{file,"src/rebar_core.erl"},{line,130}]}]}}

Now all source files are copied to .eunit.

These functions will be necessary in rebar_eunit.erl, too.