Trust the directory cache in batch mode.

When coqtop is a long-lived process (e.g. coqide), the user might be
creating files on the fly. So we have to ask the operating system whether
a file exists beforehand, so that we know whether the content of the
directory cache is outdated.

In batch mode, we can assume that the cache is always up-to-date, so we
don't need to query the operating system before trusting the content of
the cache.

On a script doing "Require Import Reals", this brings down the number of
stat syscalls from 42k to 2k. The number of syscalls could be further
halved if all_subdirs was filling the directory cache.

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