Update auto import to handle dynamic import races

[guybedford]

As discussed in #2216 (comment).

The approach taken is to wait one tick before completing the auto import. In that time the script loader path is able to then opt-out any auto imports if they have been script loaded.

@tmsns I wasn't able to replicate the race condition unfortunately, but it should now handle it correctly I believe. Any chance you could test this PR out there and confirm?

[github-actions]

core

File by file impact

File	Transform	Diff	master	auto-import-race	Event
dist/s.js	none	-175	21,829	21,654	changed
	gzip	-131	6,211	6,080
	brotli	-87	5,342	5,255
dist/s.min.js	none	+51	6,562	6,613	changed
	gzip	+57	2,601	2,658
	brotli	+42	2,339	2,381
dist/system.js	none	-175	32,275	32,100	changed
	gzip	-126	8,711	8,585
	brotli	-94	7,540	7,446
dist/system.min.js	none	+51	10,587	10,638	changed
	gzip	+58	4,044	4,102
	brotli	+50	3,617	3,667

extras

File by file impact

Pull request have no impact on extras files.

_{Generated by github pull request filesize impact}

[tmsns]

I did some first tests, and unfortunately the situation is worse now: the import of the static entry is being delayed.
The "tick" you implemented using setTimeout is creating a macrotask (https://javascript.info/event-loop). This task is allowing the browser to render already, deferring the import of the first entry. This kinda defeats the purpose of using a regular script element in the first place. In IE11, it seems to be even worse as the task is being pushed after all script tags of the page. :-/

Tomorrow, I'll test with the dynamic import, but it doesn't look good.