Make that xref_runner supports rebar3 compilation

[Euen]

When you don't have xref.config xref takes the compiled file form ebin/ directory as default

-spec check(check(), config()) -> [warning()].
check(Check, Config) ->
  XrefDefaults = maps:get(xref_defaults, Config, []),
  ConfigDirs = maps:get(dirs, Config, [ebin()]),
  ...

ebin() ->
  case filelib:is_dir("ebin") of
    true -> filename:absname("ebin");
    false -> filename:absname(".")
  end.

It's needed to support the rebar3 compilation without ebin/ directory

[David Cao]

Fixed with PR [#51]