Linting rule only for dev and test

Production build broke because our custom lint rule was compiled. The
credo linter is not available and not necessary in production.

Solution: create separate directory for dev tools.

app/lib/firehose/checks/no_conn_shadowing.ex

@@ -1,49 +0,0 @@
-defmodule Firehose.Checks.NoConnShadowing do
-  use Credo.Check,
-    base_priority: :normal,
-    category: :readability,
-    explanations: [
-      check: """
-      Conn shadowing (`conn = get(conn, ...)`) makes Phoenix controller tests
-      noisy. Use pipe chains instead:
-
-          body = conn |> get("/path") |> html_response(200)
-
-      Run `./refactor_conn_aliasing.sh <file>` to fix automatically.
-      """
-    ]
-
-  @http_verbs ~w(get post put patch delete head options)a
-
-  @impl true
-  def run(%SourceFile{} = source_file, params) do
-    issue_meta = IssueMeta.for(source_file, params)
-
-    source_file
-    |> Credo.Code.prewalk(&traverse(&1, &2, issue_meta))
-    |> Enum.reverse()
-  end
-
-  defp traverse(
-         {:=, meta, [{:conn, _, _}, {verb, _, [{:conn, _, _} | _]}]} = ast,
-         issues,
-         issue_meta
-       )
-       when verb in @http_verbs do
-    issue = issue_for(issue_meta, meta[:line], verb)
-    {ast, [issue | issues]}
-  end
-
-  defp traverse(ast, issues, _issue_meta) do
-    {ast, issues}
-  end
-
-  defp issue_for(issue_meta, line_no, verb) do
-    format_issue(
-      issue_meta,
-      message:
-        "Conn shadowing detected (`conn = #{verb}(conn, ...)`). Run `./refactor_conn_aliasing.sh <file>` to fix.",
-      line_no: line_no
-    )
-  end
-end