Refactor and simplify recipe DSL evaluation usages:

Remove most of recipe evaluation code in Recipe class, and rely on
Recipe::DSL to get evaluated recipes.

* Remove Recipe#evaluate call from CLI, rely on
  Recipe.evaluate_from_file to get the evaluated recipe;
* Implement Recipe.evaluate_from_file(filepath, env);
* Implement Recipe::DSL.evaluate(code, env);
* Remove code and filepath accessor on Recipe;
* Remove Recipe.from_file and Recipe#evaluate methods;
* Move task evaluations in Recipe::DSL#evaluate;
* Modify Recipe constructor so that it accepts tasks as argument.

spec/producer/core/recipe/dsl_spec.rb

@@ -8,6 +8,35 @@ module Producer::Core
     let(:env)     { double('env').as_null_object }
     subject(:dsl) { Recipe::DSL.new &code }
 
+    describe '.evaluate' do
+      let(:code) { 'nil' }
+
+      it 'builds a new DSL sandbox with given code' do
+        expect(Recipe::DSL).to receive(:new).once.with(code).and_call_original
+        Recipe::DSL.evaluate(code, env)
+      end
+
+      it 'evaluates the DSL sandbox code with given environment' do
+        dsl = double('dsl').as_null_object
+        allow(Recipe::DSL).to receive(:new) { dsl }
+        expect(dsl).to receive(:evaluate).with(env)
+        Recipe::DSL.evaluate(code, env)
+      end
+
+      it 'builds a recipe with evaluated tasks' do
+        dsl = Recipe::DSL.new('task(:some_task) { }')
+        allow(Recipe::DSL).to receive(:new) { dsl }
+        expect(Recipe).to receive(:new).with(dsl.tasks)
+        Recipe::DSL.evaluate(code, env)
+      end
+
+      it 'returns the recipe' do
+        recipe = double('recipe').as_null_object
+        allow(Recipe).to receive(:new) { recipe }
+        expect(Recipe::DSL.evaluate(code, env)).to be recipe
+      end
+    end
+
     describe '#initialize' do
       it 'assigns no task' do
         expect(dsl.instance_eval { @tasks }).to be_empty
@@ -30,7 +59,7 @@ module Producer::Core
     end
 
     describe '#tasks' do
-      let(:code) { proc { task(:some_task) } }
+      let(:code) { proc { task(:some_task) { } } }
 
       it 'returns registered tasks' do
         dsl.evaluate(env)
@@ -44,6 +73,14 @@ module Producer::Core
         expect { dsl.evaluate(env) }.to throw_symbol :recipe_code
       end
 
+      it 'evaluates the registered tasks' do
+        task = double('task')
+        allow(Task).to receive(:new) { task }
+        dsl = Recipe::DSL.new { task(:some_task) }
+        expect(task).to receive(:evaluate).with(env)
+        dsl.evaluate(env)
+      end
+
       it 'returns itself' do
         expect(dsl.evaluate(env)).to eq dsl
       end
@@ -81,11 +118,11 @@ module Producer::Core
       end
 
       describe '#task' do
-        let(:code) { proc { task(:first) { throw :first_task }; task(:last) } }
+        let(:code) { proc { task(:first) { :some_value }; task(:last) { } } }
 
         it 'register a task with its code' do
-          expect(dsl.tasks.first.name).to eq :first
-          expect { dsl.tasks.first.evaluate(env) }.to throw_symbol :first_task
+          expect(dsl.tasks.first.instance_eval { @block.call })
+            .to eq :some_value
         end
 
         it 'registers tasks in declaration order' do