MAGETWO-38838: Create composer management library and reuse it in both updater and setup wizard

- Changed COMPOSER env to --working-dir

Author: Ivan Gavryshko

app/code/Magento/Composer/MagentoComposerApplication.php

@@ -71,7 +71,6 @@ public function setConfig($pathToComposerHome, $pathToComposerJson)
         $this->composerHome = $pathToComposerHome;
 
         putenv('COMPOSER_HOME=' . $pathToComposerHome);
-        putenv('COMPOSER=' . $pathToComposerJson);
 
         $this->consoleApplication->setAutoExit(false);
         $this->configIsSet = true;
@@ -110,6 +109,8 @@ public function runComposerCommand(array $commandParams)
 
         $this->consoleApplication->resetComposer();
 
+        $commandParams['--working-dir'] = dirname($this->composerJson);
+
         $input = $this->consoleArrayInputFactory->create($commandParams);
 
         $exitCode = $this->consoleApplication->run($input, $this->consoleOutput);

dev/tests/unit/MagentoComposerApplicationTest.php

@@ -78,7 +78,7 @@ function testWrongExitCode()
 
     function testRunCommand()
     {
-        $inputData = ['command'=>'update'];
+        $inputData = ['command'=>'update', '--working-dir' => '.'];
 
         $this->application->setConfig('path1', 'path2');
         $this->composerApplication->expects($this->once())->method('resetComposer');