Fixup and simplify auto import cases

[guybedford]

This fixes the issues pointed out in #2223 (comment), where the timeout is triggering after the ready state has changed thus cancelling an in-progress auto import.

It turns out the timeout mechanism is actually unnecessary as the process of simply checking the registration to clear it in the dynamic case is all that is needed.

The other mechanism to clear the auto import registry entirely has been removed as it's mostly a GC optimization for some impractical edge cases, whereas most proper usage would never hit those. So it also removes some bytes.

[github-actions]

core

File by file impact

File	Transform	Diff	master	auto-import-fixes	Event
dist/s.js	none	-341	21,896	21,555	changed
	gzip	-77	6,198	6,121
	brotli	-72	5,354	5,282
dist/s.min.js	none	-98	6,659	6,561	changed
	gzip	-39	2,681	2,642
	brotli	-35	2,401	2,366
dist/system.js	none	-341	32,342	32,001	changed
	gzip	-63	8,697	8,634
	brotli	-59	7,542	7,483
dist/system.min.js	none	-98	10,684	10,586	changed
	gzip	-39	4,125	4,086
	brotli	-31	3,685	3,654

extras

File by file impact

Pull request have no impact on extras files.

_{Generated by github pull request filesize impact}

[guybedford]

Released in 6.4.2.