Use LFE from hex

[arpunk]

This enables us to use the rebar3 hex cache instead of cloning the LFE repo all the time.

[yurrriq]

IIRC @oubiwann wants to make Hex adoption optional. I have a workaround somewhere that uses an env var like LFE_HEX=1 to enable Hex and a rebar.config.script to match. There was a problem with rebar3_hex that's since been addressed, so my script should work now. I'll try to track it down soon. Feel free to ping me if I forget.

[arpunk]

I see. Hex is becoming an standard in the Erlang ecosystem so it's easier for us to have github/gitlab as the optional choice and get the users fetch from Hex instead. I've never used gitlab but I rely on Hex for my Erlang/Elixir projects (and LFE ones aswell).

I'm ok if this is not a desired approach.