BC break: removed deprecated `PackageVersions\Versions::ROOT_PACKAGE_…

…NAME` constant

This simplifies the tool to no longer be a plugin: `PackageVersions\Versions` is now a much simpler
class that is no longer generated/written to disk at installation, and `ocramius/package-versions`
is no longer a `"type": "composer-plugin"`, but rather a more usual `"type": "library"` package.

This:

 * Fixes #138 - this library no longer changes `vendor` post-installation
 * Fixes #142 - the `rootPackage` version is now detected via composer, and no longer leads to
   changes in `vendor/ocramius/package-versions/src/PackageVersions/Versions.php` at each change
   of source root in a project
 * Fixes #152 - when `"lock": false` is used in composer, since we no longer access lock file
   information from sources of this package
 * Fixes #107 - writing to `vendor` is no longer happening from this library, so no file access
   rights should be needed.

src/PackageVersions/Versions.php

@@ -17,12 +17,6 @@
  */
 final class Versions
 {
-    /**
-     * @deprecated please use {@see self::rootPackageName()} instead.
-     *             This constant will be removed in version 2.0.0.
-     */
-    public const ROOT_PACKAGE_NAME = 'unknown/root-package@UNKNOWN';
-
     private function __construct()
     {
     }
@@ -48,13 +42,6 @@ public static function rootPackageName() : string
      */
     public static function getVersion(string $packageName) : string
     {
-        /** @psalm-suppress DeprecatedConstant */
-        if ($packageName === self::ROOT_PACKAGE_NAME) {
-            $rootPackage = InstalledVersions::getRootPackage();
-
-            return $rootPackage['pretty_version'] . '@' . $rootPackage['reference'];
-        }
-
         return InstalledVersions::getPrettyVersion($packageName)
             . '@' . InstalledVersions::getReference($packageName);
     }