Extracting logic into a new class related to selecting the latest version

Also refactored InitCommand slightly so that you can use this "latest version"
functionality when searching for a package as well.

tests/Composer/Test/Package/Version/VersionSelectorTest.php

@@ -0,0 +1,71 @@
+<?php
+
+/*
+ * This file is part of Composer.
+ *
+ * (c) Nils Adermann <naderman@naderman.de>
+ *     Jordi Boggiano <j.boggiano@seld.be>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Composer\Test\Package\Version;
+
+use Composer\Package\Version\VersionSelector;
+
+class VersionSelectorTest extends \PHPUnit_Framework_TestCase
+{
+    // A) multiple versions, get the latest one
+    // B) targetPackageVersion will pass to pool
+    // C) No results, throw exception
+
+    public function testLatestVersionIsReturned()
+    {
+        $packageName = 'foobar';
+
+        $package1 = $this->createMockPackage('1.2.1');
+        $package2 = $this->createMockPackage('1.2.2');
+        $package3 = $this->createMockPackage('1.2.0');
+        $packages = array($package1, $package2, $package3);
+
+        $pool = $this->createMockPool();
+        $pool->expects($this->once())
+            ->method('whatProvides')
+            ->with($packageName, null, true)
+            ->will($this->returnValue($packages));
+
+        $versionSelector = new VersionSelector($pool);
+        $best = $versionSelector->findBestCandidate($packageName);
+
+        // 1.2.2 should be returned because it's the latest of the returned versions
+        $this->assertEquals($package2, $best, 'Latest version should be 1.2.2');
+    }
+
+    public function testFalseReturnedOnNoPackages()
+    {
+        $pool = $this->createMockPool();
+        $pool->expects($this->once())
+            ->method('whatProvides')
+            ->will($this->returnValue(array()));
+
+        $versionSelector = new VersionSelector($pool);
+        $best = $versionSelector->findBestCandidate('foobaz');
+        $this->assertFalse($best, 'No versions are available returns false');
+    }
+
+    private function createMockPackage($version)
+    {
+        $package = $this->getMock('\Composer\Package\PackageInterface');
+        $package->expects($this->any())
+            ->method('getVersion')
+            ->will($this->returnValue($version));
+
+        return $package;
+    }
+
+    private function createMockPool()
+    {
+        return $this->getMock('Composer\DependencyResolver\Pool', array(), array(), '', true);
+    }
+}