WIP: ext_mod: Add support to download dependencies using Hex when ava…

…ilable

Set the desired hex package version in the module's rebar.config

For example, to fetch hex package recon 2.5.5 when Hex is available,
and otherwise download using git, in the file
  ejabberd-contrib/ejabberd_observer_cli/rebar.config
set both the hex version and git details:
{deps, [
        {recon, "2.5.5", {git, "https://github.com/ferd/recon"}}
       ]}.

src/ext_mod.erl

@@ -762,8 +762,10 @@ fetch_rebar_deps(SrcDir) ->
             {ok, CurDir} = file:get_cwd(),
             file:set_cwd(SrcDir),
             filelib:ensure_dir(filename:join("deps", ".")),
-            lists:foreach(fun({_App, Cmd}) ->
-                        os:cmd("cd deps; "++Cmd++"; cd ..")
+            lists:foreach(fun({App, Cmd}) ->
+                        io:format("Fetching dependency ~s: ", [App]),
+                        Result = os:cmd("cd deps; "++Cmd++"; cd .."),
+                        io:format("~s", [Result])
                 end, Deps),
             file:set_cwd(CurDir)
     end.
@@ -777,6 +779,28 @@ rebar_deps(Script) ->
         _ ->
             []
     end.
+
+rebar_dep({App, Version, Git}) when Version /= ".*" ->
+    AppS = atom_to_list(App),
+    %%
+    %% Work in Progress trying to get the feature work when compiled with mix and running OTP release:
+    %%
+    %BootPath = filename:rootname(filelib:wildcard("../releases/*/start_clean.boot")),
+    %RL = "../lib",
+    %Help = os:cmd("iex --boot " ++ BootPath ++ " -S mix hex.package"),
+    %Help = os:cmd("iex --boot " ++ BootPath ++ " --boot-var RELEASE_LIB "++RL++" -S mix hex.package"),
+    %Help = os:cmd("iex --boot " ++ BootPath ++ " --boot-var RELEASE_LIB ../lib/ -S mix hex.package"),
+    %
+    Help = os:cmd("mix hex.package"),
+    case string:find(Help, "mix hex.package fetch") /= nomatch of
+        true ->
+            {App, "mix hex.package fetch "++AppS++" "++Version++" --unpack"};
+        false ->
+            io:format("I'll download ~p using git because I can't use Mix "
+                      "to fetch from hex.pm:~n~s", [AppS, help]),
+            rebar_dep({App, ".*", Git})
+    end;
+
 rebar_dep({App, _, {git, Url}}) ->
     {App, "git clone "++Url++" "++filename:basename(App)};
 rebar_dep({App, _, {git, Url, {branch, Ref}}}) ->