Add option to disable the lock file

When the `lock` option is set to false, composer will not write a
`composer.lock` file to disk. This signals that the package is meant
to be developed with unlocked and always updated dependencies. At the
moment, both `install` and `update` are allowed to install the
dependencies for such a package. If #6822 is implemented, only `update`
should be used for packages without a lockfile.

https://github.com/composer/composer/issues/8354

doc/06-config.md

@@ -296,4 +296,9 @@ Example:
 Defaults to `true`. If set to `false`, Composer will not create `.htaccess` files
 in the composer home, cache, and data directories.
 
+## lock
+
+Defaults to `true`. If set to `false`, Composer will not create a `composer.lock` 
+file.
+
 ← [Repositories](05-repositories.md)  |  [Community](07-community.md) →

res/composer-schema.json

@@ -290,6 +290,10 @@
                 "sort-packages": {
                     "type": "boolean",
                     "description": "Defaults to false. If set to true, Composer will sort packages when adding/updating a new dependency."
+                },
+                "lock": {
+                    "type": "boolean",
+                    "description": "Defaults to true. If set to false, Composer will not create a composer.lock file."
                 }
             }
         },

src/Composer/Command/ConfigCommand.php

@@ -412,6 +412,7 @@ EOT
             ),
             'github-expose-hostname' => array($booleanValidator, $booleanNormalizer),
             'htaccess-protect' => array($booleanValidator, $booleanNormalizer),
+            'lock' => array($booleanValidator, $booleanNormalizer),
         );
         $multiConfigValues = array(
             'github-protocols' => array(

src/Composer/Config.php

@@ -63,6 +63,7 @@ class Config
         'archive-dir' => '.',
         'htaccess-protect' => true,
         'use-github-api' => true,
+        'lock' => true,
         // valid keys without defaults (auth config stuff):
         // bitbucket-oauth
         // github-oauth
@@ -329,6 +330,8 @@ class Config
                 return $this->config[$key] !== 'false' && (bool) $this->config[$key];
             case 'use-github-api':
                 return $this->config[$key] !== 'false' && (bool) $this->config[$key];
+            case 'lock':
+                return $this->config[$key] !== 'false' && (bool) $this->config[$key];
             default:
                 if (!isset($this->config[$key])) {
                     return null;

src/Composer/Installer.php

@@ -118,7 +118,7 @@ class Installer
     protected $preferStable = false;
     protected $preferLowest = false;
     protected $skipSuggest = false;
-    protected $writeLock = true;
+    protected $writeLock;
     protected $executeOperations = true;
 
     /**
@@ -164,6 +164,8 @@ class Installer
         $this->installationManager = $installationManager;
         $this->eventDispatcher = $eventDispatcher;
         $this->autoloadGenerator = $autoloadGenerator;
+
+        $this->writeLock = $config->get('lock');
     }
 
     /**