Application not getting loaded

Hello everyone.

I am experiencing an issue, which I am pretty sure is caused just by me not understanding stuff. Which is usual

I am currently writing a small package. Its only purpose is providing a few helper mix tasks. I want them to be configurable by the user through his Config but provide some defaults as well.
The issue is that even though I specified :env: key in the &application/0 in mix.exs the defaults are still missing and all of my Application.compile_env calls are failing obviously.

I traced the issue to my application not getting loaded. But why? I don’t understand why the hell application that I am trying to compile is not getting loaded?

Elixir version is 1.10.3 compiled with erlang 23.
important contents of the mix.exs:

defmodule FooBar.MixProject do
  use Mix.Project
  def project do
    [
      app: :foobar,
      version: "0.1.0",
      elixir: "~> 1.10"
    ]
  end

  def application do
    [
      extra_applications: [:logger],
      env: [
        foo: "bar"
      ]
    ]
  end
end

somewhere in the project:

defmodule FooBar.Spam do
  @foo Application.compile_env!(:foobar, :foo)
end

and when I run mix compile I get an error:

** (ArgumentError) could not fetch application environment :foo for application :foobar because the application was not loaded/started. If your application depends on :foobar at runtime, make sure to load/start it or list it under :extra_applications in your mix.exs file

So, the issue is pretty clear - application is not loaded. And if I load it manually when standing on pry it works.
My question is why it is not loaded by default? If I should load it myself, where is the best place to do so?