rebar_core: consistently order args and simplify code

* Fix arg order:
  The order of arguments got inconsistent over time. To fix that, use
  the same consistent order in all functions.

* Avoid one erlang:'++'/2 call in process_dir/6.

* Avoid lists:prefix/2 and atom_to_list/1 calls:
  We can easily avoid 2 lists:prefix/2 calls and one atom_to_list/1 call
  in execute/5 by passing in whether the command is a hook or not. The
  resulting code is simpler and more readable.
Fix #267 (code path regression)

Since the introduction of -r/--recursive, deps were not properly added
to the code path when running ct, eunit, etc.

To fix that, pass a flag down to process_dir1 and conditionalize
execution of the command. This moves the decision into process_dir1,
where we can choose to invoke preprocess/2 and postprocess/2 but not
execute the command.

Without this fix, you'd have to, for example, invoke 'rebar -r ct
skip_deps=true', if you wanted to run base_dir's ct suites with deps on
the code path (while skipping all non-base_dir ct suites).

So, with this patch applied, if you run
$ rebar ct
deps will be on the code path, and only base_dir's ct suites will be
tested.

If you want to test ct suites in base_dir and sub_dirs, you have to run
$ rebar -r ct skip_deps=true

If you want to test ct suites in all dirs, you have to run
$ rebar -r ct

The fix is not specific to ct and applies to all commands.