Make HTTPoison adapter compile only when HTTPoison is loaded

The adapter module references %HTTPoison.Response{} and %HTTPoison.Error{}
structs at compile time, so it could not be compiled in a consumer
project that drops httpoison from its deps — even though mix.exs already
declares httpoison as optional: true.

Wrap the defmodule block in Code.ensure_loaded?(HTTPoison) so the module
is only defined when HTTPoison is actually available. Funkspector's own
test matrix still pulls HTTPoison transitively (optional deps are
included in the package's own build), so test.adapters keeps exercising
both adapters.

Author: jaimeiniesta

lib/funkspector/http/adapters/httpoison.ex

@@ -1,8 +1,17 @@
+if Code.ensure_loaded?(HTTPoison) do
 defmodule Funkspector.HTTP.Adapters.HTTPoison do
   @moduledoc """
   Opt-in Funkspector HTTP adapter, backed by
   [HTTPoison](https://hex.pm/packages/httpoison) on top of hackney.
 
+  Compiled only when the `:httpoison` dependency is actually present in the
+  consuming project — `httpoison` is declared `optional: true` in
+  `funkspector`'s `mix.exs`, so projects that stick with the default Req
+  adapter do not need to pull it in. When HTTPoison is missing the module
+  is simply not defined; selecting it as the adapter in that case fails
+  with `UndefinedFunctionError` at call time, which is the desired
+  behaviour for an explicit opt-in.
+
   This is the historical Funkspector HTTP path, preserved for users who
   cannot — or do not want to — move to Req yet. The defaults here match
   the pre-2.0 behavior:
@@ -101,3 +110,4 @@ defmodule Funkspector.HTTP.Adapters.HTTPoison do
     end
   end
 end
+end