fingerprint: Recover from fs::stat errors

Fingerprinting will fail at an fs::stat() call if there is a broken
symlink in a package's directory.  This commit recovers from fs::stat()
errors on broken symlinks by treating them as having mtime 0, which
should not affect the overall fingerprint.

Fix #135
auto merge of #139 : tomjakubowski/cargo/fix-symlink-blowup, r=alexcrichton

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