Optimize virtual-target.register for the case of many identically named

targets. In that case, we used to create a long list of targets and searched
it whenever new target is registered. Now the list is for each path/name
combination, so is much shorter.


[SVN r31054]

v2/build/virtual-target.jam

@@ -813,7 +813,7 @@ rule from-file ( file : file-loc : project )
 rule register ( target )
 {
     local signature = [ sequence.join
-       [ $(target).name ] : - ] ;    
+       [ $(target).path ] [ $(target).name ] : - ] ;    
 
             
     local result ;