Don't use BasePackage::$stabilities in VersionParser

Working towards #3545.

This adds VersionParser::$stabilities, which is an array of the
supported stability levels. This adds a little bit of duplication,
however we were already hardcoding the various stabilities in regular
expressions and functions like parseStability().

src/Composer/Package/Version/VersionParser.php

@@ -12,7 +12,6 @@
 
 namespace Composer\Package\Version;
 
-use Composer\Package\BasePackage;
 use Composer\Package\PackageInterface;
 use Composer\Package\Link;
 use Composer\Package\LinkConstraint\EmptyConstraint;
@@ -28,6 +27,10 @@ class VersionParser
 {
     private static $modifierRegex = '[._-]?(?:(stable|beta|b|RC|alpha|a|patch|pl|p)(?:[.-]?(\d+))?)?([.-]?dev)?';
 
+    private static $stabilities = array(
+        'stable', 'RC', 'beta', 'alpha', 'dev',
+    );
+
     /**
      * Returns the stability of a version
      *
@@ -241,7 +244,7 @@ class VersionParser
     {
         $prettyConstraint = $constraints;
 
-        if (preg_match('{^([^,\s]*?)@('.implode('|', array_keys(BasePackage::$stabilities)).')$}i', $constraints, $match)) {
+        if (preg_match('{^([^,\s]*?)@('.implode('|', self::$stabilities).')$}i', $constraints, $match)) {
             $constraints = empty($match[1]) ? '*' : $match[1];
         }
 
@@ -284,7 +287,7 @@ class VersionParser
 
     private function parseConstraint($constraint)
     {
-        if (preg_match('{^([^,\s]+?)@('.implode('|', array_keys(BasePackage::$stabilities)).')$}i', $constraint, $match)) {
+        if (preg_match('{^([^,\s]+?)@('.implode('|', self::$stabilities).')$}i', $constraint, $match)) {
             $constraint = $match[1];
             if ($match[2] !== 'stable') {
                 $stabilityModifier = $match[2];