'Erlang' projects that do not contain any erlang files (Joxa, LFE,
Elixir, etc) break the eunit task. It attempts to copy an empty list
of source files to the .eunit directory. This change makes copying an
empty list a simple no-op.

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.