Handle Schemaless Proxy URLs in ENV vars

We've had multiple tickets opened because of unclear PROXY settings when
the scheme is missing form the URI. To be helpful, we instead add them
dynamically whenever they're missing.

Example issues:
- https://github.com/erlang/rebar3/issues/1747
- https://github.com/erlang/rebar3/issues/1697

src/rebar_utils.erl

@@ -854,10 +854,18 @@ set_httpc_options(_, []) ->
     ok;
 
 set_httpc_options(Scheme, Proxy) ->
-    {ok, {_, UserInfo, Host, Port, _, _}} = http_uri:parse(Proxy),
+    URI = normalise_proxy(Scheme, Proxy),
+    {ok, {_, UserInfo, Host, Port, _, _}} = http_uri:parse(URI),
     httpc:set_options([{Scheme, {{Host, Port}, []}}], rebar),
     set_proxy_auth(UserInfo).
 
+normalise_proxy(Scheme, URI) ->
+    case re:run(URI, "://", [unicode]) of
+        nomatch when Scheme =:= https_proxy -> "https://" ++ URI;
+        nomatch when Scheme =:= proxy -> "http://" ++ URI;
+        _ -> URI
+    end.
+
 url_append_path(Url, ExtraPath) ->
      case http_uri:parse(Url) of
          {ok, {Scheme, UserInfo, Host, Port, Path, Query}} ->