app: give credo checks one load path (fixes redefining-module warning)

lib_dev/ was both compiled (elixirc_paths for :dev and :test) and source-loaded
by credo through `requires:` in .credo.exs. Two load paths for one module means
that whenever _build/dev already holds the beam, `mix credo` warned:

    warning: redefining module Firehose.Checks.NoConnShadowing
    (current version loaded from _build/dev/.../Elixir.Firehose.Checks.NoConnShadowing.beam)

Keep the explicit contract and drop the accidental one: `requires:` is the
documented way for credo to load project checks, so lib_dev/ now leaves
elixirc_paths in every environment. Nothing under test/ referenced the check, so
the :test entry was dead as well, and credo compiles the file on every run
anyway, so the check still gets compiled — just once.

One-time migration for checkouts that already built it:
`mix clean` (or delete _build/dev) to drop the stale beam; until then the old
warning persists. Fresh clones are unaffected.

app/mix.exs

@@ -33,8 +33,15 @@ defmodule Firehose.MixProject do
   end
 
   # Specifies which paths to compile per environment.
-  defp elixirc_paths(:test), do: ["lib", "lib_dev", "test/support"]
-  defp elixirc_paths(:dev), do: ["lib", "lib_dev"]
+  #
+  # `lib_dev/` is deliberately NOT compiled in any environment: it holds
+  # credo-only checks (Firehose.Checks.NoConnShadowing), and credo loads them
+  # itself from source via `requires:` in .credo.exs. Compiling that path as
+  # well gave every run two load paths for the same module, which surfaces as
+  # "warning: redefining module Firehose.Checks.NoConnShadowing" on a warm
+  # `_build/dev`. Keep the single source load: if you add code under lib_dev/
+  # that is NOT a credo check, give it its own compiled path instead.
+  defp elixirc_paths(:test), do: ["lib", "test/support"]
   defp elixirc_paths(_), do: ["lib"]
 
   # Specifies your project dependencies.