realpath() does not gracefully continue when an input file does not exist

[mcauley-penney]

Problem: The realpath fn is used to resolve paths so that we can deduplicate paths that are given with different forms, e.g. we want to remove duplicates even in situations when a relative and absolute path are given for the same item. As of now, mmv segfaults when given a path that does not exist.

Proposed solution: Continue when a path doesn't exist. We can use this an opportunity to filter inputs that aren't real. This may not be a small edge case either: I found this by accidentally using mmv like mv: I provided a destination name that didn't exist in my file system. This is probably a common mistake for command line users, I do this kind of regularly.