Prune unreachable required versions correctly for aliased packages

In trials this seems pointless, so maybe better to skip aliases and
reduce memory and cpu wasted on looking these things up

src/Composer/DependencyResolver/PoolBuilder.php

@@ -99,12 +99,27 @@ class PoolBuilder
         }
 
         foreach ($this->packages as $i => $package) {
-            if (!$package instanceof AliasPackage && !isset($this->aliasMap[spl_object_hash($package)]) && isset($this->nameConstraints[$package->getName()])) {
+            // we check all alias related packages at once, so no need ot check individual aliases
+            // isset also checks non-null value
+            if (!$package instanceof AliasPackage && isset($this->nameConstraints[$package->getName()])) {
                 $constraint = $this->nameConstraints[$package->getName()];
 
-                if ($constraint && !$constraint->matches(new Constraint('==', $package->getVersion()))) {
-                    unset($this->packages[$i]);
-                    unset($this->priorities[$i]);
+                $aliasedPackages = array($i => $package);
+                if (isset($this->aliasMap[spl_object_hash($package)])) {
+                    $aliasedPackages += $this->aliasMap[spl_object_hash($package)];
+                }
+
+                $found = false;
+                foreach ($aliasedPackages as $packageOrAlias) {
+                    if ($constraint->matches(new Constraint('==', $packageOrAlias->getVersion()))) {
+                        $found = true;
+                    }
+                }
+                if (!$found) {
+                    foreach ($aliasedPackages as $index => $packageOrAlias) {
+                        unset($this->packages[$index]);
+                        unset($this->priorities[$index]);
+                    }
                 }
             }
         }
@@ -120,16 +135,21 @@ class PoolBuilder
 
         $this->pool->setPackages($this->packages, $this->priorities);
 
+        unset($this->aliasMap);
+        unset($this->loadedNames);
+        unset($this->nameConstraints);
+
         return $this->pool;
     }
 
     private function loadPackage(PackageInterface $package, $repoIndex)
     {
+        $index = count($this->packages);
         $this->packages[] = $package;
         $this->priorities[] = -$repoIndex;
 
         if ($package instanceof AliasPackage) {
-            $this->aliasMap[spl_object_hash($package->getAliasOf())][] = $package;
+            $this->aliasMap[spl_object_hash($package->getAliasOf())][$index] = $package;
         }
 
         // handle root package aliases
@@ -147,7 +167,7 @@ class PoolBuilder
             $package->getRepository()->addPackage($aliasPackage); // TODO do we need this?
             $this->packages[] = $aliasPackage;
             $this->priorities[] = -$repoIndex;
-            $this->aliasMap[spl_object_hash($aliasPackage->getAliasOf())][] = $aliasPackage;
+            $this->aliasMap[spl_object_hash($aliasPackage->getAliasOf())][$index+1] = $aliasPackage;
         }
 
         $loadNames = array();