Allow multiple formula to symlink the same directory

During the link step, if the destination symlink already exists, unlink it, and create a directory instead, then relink the original contents. Then continue linking the formula in question.

Fixes Homebrew/homebrew#62

Library/Homebrew/keg.rb

@@ -28,6 +28,15 @@ class Keg <Pathname
     raise "#{to_s} is not a directory" unless directory?
   end
 
+  # if path is a file in a keg then this will return the containing Keg object
+  def self.for path
+    while not path.root?
+      return Keg.new(path) if path.parent.parent == HOMEBREW_CELLAR
+      path = path.parent.realpath # realpath() prevents root? failing
+    end
+    raise "#{path} is not inside a keg"
+  end
+
   def uninstall
     chmod_R 0777 # ensure we have permission to delete
     rmtree
@@ -72,28 +81,50 @@ class Keg <Pathname
     return $n+$d
   end
 
-private
+protected
+  def resolve_any_conflicts dst
+    # if it isn't a directory then a severe conflict is about to happen. Let
+    # it, and the exception that is generated will message to the user about
+    # the situation
+    if dst.symlink? and dst.directory?
+      src = (dst.parent+dst.readlink).cleanpath
+      keg = Keg.for(src)
+      dst.unlink
+      keg.link_dir(src) { :mkpath }
+      return true
+    end
+  end
+
   # symlinks the contents of self+foo recursively into /usr/local/foo
   def link_dir foo
-    root=self+foo
+    root = self+foo
 
     root.find do |src|
       next if src == root
 
-      dst=HOMEBREW_PREFIX+src.relative_path_from(self)
+      dst = HOMEBREW_PREFIX+src.relative_path_from(self)
       dst.extend ObserverPathnameExtension
 
       if src.file?
         dst.make_relative_symlink src
       elsif src.directory?
+        # if the dst dir already exists, then great! walk the rest of the tree tho
+        next if dst.directory? and not dst.symlink?
+
         # no need to put .app bundles in the path, the user can just use
         # spotlight, or the open command and actual mac apps use an equivalent
         Find.prune if src.extname.to_s == '.app'
 
         case yield src.relative_path_from(root)
-          when :skip then Find.prune
-          when :mkpath then dst.mkpath
-          else dst.make_relative_symlink src; Find.prune
+        when :skip
+          Find.prune
+        when :mkpath
+          dst.mkpath unless resolve_any_conflicts(dst)
+        else
+          unless resolve_any_conflicts(dst)
+            dst.make_relative_symlink(src)
+            Find.prune
+          end
         end
       end
     end