Fixing the contention condition caused by RegisterResolversManifests

[SimaTian]

Fixes #11043, #7927

Context

There was a contention condition described here:
One thread enters and locks, then initializes a list, starts pushing things onto the list, which is now no longer null.
Second thread then checks, sees the list is not empty and bypasses the lock, acquires enumerator.
First thread pushes additional item into the list.
Second thread throws.

Changes Made

Initializing the list to a placeholder first, only assigning it when it is finished. Due to this, until the list is fully initialized, all threads have to enter the lock, then they will check that the list is now initialized and return.
This will result in some minor locking overhead, but it is not as strict as locking the collection for all reads.

Furthermore, since the collection is supposed to be read only as written here, I've made it into IReadOnlyList instead of the current IList.

Testing

I got a reproduction, although a bit clunky.
First commit checks should fail. Then I will push the fix which should succeed.

[donJoseLuis]

few minor comments; man one moving away from using the thread class

[JanKrivanek]

First quick pass.

There is lots of test related code in production code - can we isolate it inot tests

[JanKrivanek]

Thank you for making the efforts to clean the production part of the code! I like this!!

[KalleOlaviNiemitalo]

Fixes #11043, #7927

GitHub did not automatically close #7927. See "Multiple issues" in https://docs.github.com/en/issues/tracking-your-work-with-issues/using-issues/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword.