automata: make behavior more consistent when WhichCaptures::None is used

[BurntSushi]

Specifically, when used with meta::Regex. Before this PR, if callers
built a meta::Regex with WhichCaptures::None, then it was possible
for find to sometimes return Some and sometimes return None,
just based on the sequence of previous search calls.

In particular, when WhichCaptures::None is used, some regex engines
(like the PikeVM) cannot report match offsets while some (like the
lazy DFA) can. This meant that if the meta regex engine happened to
select the lazy DFA for a Regex::find call, then it would return
Some. But if it happened to select the Pike VM, then it would return
None. Since engine selection can be influenced by the haystack itself,
this leads to the behavior of find being tied to the contents of the
haystack.

Instead, what we should do is make it so anything that returns match
offsets on a meta::Regex will always return None when
WhichCaptures::None is used, even if Regex::is_match returns
true.

(Yes, this is a weird option and it's crazy that Regex::is_match can
return true while Regex::find can return None. This was already
true before this PR and is a result of a very low level option that
optimizes for memory usage in specific circumstances. This sort of
whacky behavior can't be observed in the regex crate API. Only in
regex-automata.)