Drop the leading '/' from the prefix

The prefix, if not empty, would contain a leading slash. The `OpamPath` functions then would use this prefix with the `/` operator yielding paths with double slashes in them. This just drops the first character of the prefix if it's not empty. This first character should always be a slash unless the prefix directory is somehow a prefix of a sibling directory name. I don't think this condition can ever occur, however.