fingerprint: Recover from fs::stat errors

Fingerprinting will fail at an `fs::stat()` call if there is a symlink in
a package's directory pointing to a non-existent file or directory.
This commit recovers from an `fs::stat(`) error on these bogus symlinks by
faking an mtime of 0, which should not affect the overall fingerprint.

Fix #135
Could you add a comment here as well explaining why we're ignoring errors from `stat`?