clarify use of ns in packages - clojure

Question

Started playing with Clojure yesterday.
I can't get around how the module system works:
I have installed cursive
I have created a project following the leiningen template
I have two clojure files under /src/clojure_first_steps
core.clj
(ns clojure-first-steps.core)
(:require [clojure-first-steps.utils :refer :all])
(defn run-other-foo
(foo-2 ["hello"]))
utils.clj
(ns clojure-first-steps.utils)
(defn foo-2 [x] (x))
although 'lein compile' runs without probs, 'lein test' fails to compile on
(:require [clojure-first-steps.utils :refer :all]), the test being:
(ns clojure-first-steps.core-test
(:require [clojure.test :refer :all]
[clojure-first-steps.core :refer :all]))
(deftest a-test
(testing "I can access dependecies from another module"
(is (= "hello" (run-other-foo)))))
The error message is java.lang.ClassNotFoundException: clojure-first-steps.utils
EDIT: Project tree
.
├── CHANGELOG.md
├── clojure_first_steps.iml
├── doc
│   └── intro.md
├── LICENSE
├── project.clj
├── README.md
├── resources
├── src
│   ├── clojure_first_steps
│   │   ├── core.clj
│   │   └── utils.clj
├── target
│   ├── classes
│   │   └── META-INF
│   │   └── maven
│   │   └── clojure_first_steps
│   │   └── clojure_first_steps
│   │   └── pom.properties
│   ├── repl-port
│   └── stale
│   └── leiningen.core.classpath.extract-native-dependencies
└── test
├── clojure_first_steps
│   └── core_test.clj

Answer 1

In your core.clj:
(ns clojure-first-steps.core)
(:require [clojure-first-steps.utils :refer :all])
This is incorrect - the (:require) clause needs to be inside the ns macro. Because it is not, the symbols in the vector are looked up (and obviously not found).
(ns clojure-first-steps.core
(:require [clojure-first-steps.utils :refer :all]))
This tells the Clojure compiler to load clojure-first-steps.utils (if it has not already), and refer it's definitions in your newly created namespace.