Check if paths may already include executable

The current convert-path-finding logic blindly appends the `convert_name` (the name of the executable) to the end of every path before checking it, but we give some examples where it is already on the end, and many users may logically think this is valid because `convert_path` is ambiguous (it's not `convert_directory`).

This change suppresses concatenation for path strings that already end with the executable name.