Smarter ambiguity checks in extract

[Peter Hosey]

Two ways extract could be smarter in when it refuses to do an extraction because the name requested is ambiguous:

• Ignore, or at least disfavor, alias files. If there are still multiple candidates after ignoring aliases, then the request is ambiguous. If there are multiple candidates that are all aliases, then the request is ambiguous. But if there is only one candidate that isn't an alias, regardless of how many matches there are that are aliases, then the one non-alias candidate is an unambiguous match.
• If one of the candidates is a folder that contains (all of) the other candidate(s), maybe we should prefer the folder. (The folder might be the root of the volume. For example, consider a boot disk named “System”—the user could simply not specify a name if they want to extract the whole volume.) I'm less sure of this one; it might still be a case where the right answer is for the user to disambiguate explicitly by path.