rebar_ct: fix Dialyzer warning caused by bcc57e9b

Also fix a single line exceeding max column.
rebar_core: fix consistency issues caused by 252b31f

- refactor plugin dirs code to be simpler and easier to read
- use erlang-mode's default (%%) comments for portability/consistency
- make sure erlang-mode's indenter is used so that a future whole
  buffer indent doesn't get messed up
Delete dialyzer_warnings on 'make distclean'
Update THANKS file
rebar_xref: regression fixes and updates for a5be40c96

- restore support for "rebar help xref"
- update rebar.config.sample
- update 'help xref' string
- simplify new/changed functions by breaking out code or
  using simpler syntax where applicable