Warn only if snapshots are found in hidden or ignored locations

[mitsuhiko]

This changes the current warning that comes up whenever ignored or hidden paths exists to one that only comes up if actual snapshots are found in those locations.

Refs #329

[mitsuhiko]

Before merging this i also want to make this behavior entirely optional so that it can be disabled if it causes an unnecessary performance burden.

[max-sixty]

Thank you v much

One sample of performance, from #319 (comment):

Without ignoring, it is much slower — at least on prql, 3.69s vs. 4.15s with --no-ignore for cargo insta test, including the fixed costs of running the tests.

I guess because:

❯ fd | wc -l
     683

❯ fd -uu | wc -l
  101571

[max-sixty]

(as per that issue, checking when writing would get around the perf implications, but be more work, and fail to catch snapshots that are ignored after being written)