a.ex

defmodule A do
  use Mockery

  @b Mockery.of(B)
  @c C
  @d Mockery.of("D")

  def run do
    @b.fun()
    @c.fun()
    @d.fun()
    E.fun()
  end
end

b.ex

defmodule B do
  def fun, do: 42
end

c.ex

defmodule C do
  def fun, do: 42
end

d.ex

defmodule D do
  def fun, do: 42
end

e.ex

defmodule E do
  def fun, do: 42
end

mix_xref.txt

⌘ ~/code/sandbox (master) λ mix xref graph
lib/a.ex
├── lib/b.ex (compile)
├── lib/c.ex (compile)
├── lib/mockery.ex (compile)
├── lib/d.ex
└── lib/e.ex

mockery.ex

defmodule Mockery do
  defmacro __using__(_opts) do
    quote do
      require Mockery
    end
  end

  defmacro of(module) when is_binary(module) do
    mod = Module.concat([module])
    quote do: unquote(mod)
  end

  defmacro of(module) do
    quote do: unquote(module)
  end
end