[bug] rebar_mix can not load eex, iex ...

[starbelly]

Reported by tclugao originally in : Supersonido/rebar_mix#33

Elixir Version: 1.13.3-otp-22
rebar_mix: 0.5.0 or 0.5.1

and I also add elixir in rebar.config, when I run rebar3 as test shell, and rp(code:get_path()). , will miss some elixir libs,only load the following libs:
"/home/tt_gao/.asdf/installs/elixir/1.13.3-otp-22/bin/../lib/elixir/../logger/ebin",
"/home/tt_gao/.asdf/installs/elixir/1.13.3-otp-22/bin/../lib/elixir/../mix/ebin",
"/home/tt_gao/.asdf/installs/elixir/1.13.3-otp-22/bin/../lib/elixir/../elixir/ebin",

and when I change into rebar_mix version as 0.4.0, it is ok, as following:
"/home/tt_gao/.asdf/installs/elixir/1.13.3-otp-22/bin/../lib/elixir/../eex/ebin",
"/home/tt_gao/.asdf/installs/elixir/1.13.3-otp-22/bin/../lib/elixir/../ex_unit/ebin",
"/home/tt_gao/.asdf/installs/elixir/1.13.3-otp-22/bin/../lib/elixir/../mix/ebin",
"/home/tt_gao/.asdf/installs/elixir/1.13.3-otp-22/bin/../lib/elixir/../logger/ebin",
"/home/tt_gao/.asdf/installs/elixir/1.13.3-otp-22/bin/../lib/elixir/../iex/ebin",
"/home/tt_gao/.asdf/installs/elixir/1.13.3-otp-22/bin/../lib/elixir/../elixir/ebin"