Make relocatable bottles more robust

There are some valid installations that still break them.

For example: https://github.com/mxcl/homebrew/issues/24764#issuecomment-29761016

I think that as part of bottling, all references to the cellar should be replaced with `@@CELLAR@@`, and all references to the prefix with `@@PREFIX@@`. This would resolve the ambiguities that the current relocation code faces.