Restore ordered delivery of repository and registry delegate callbacks

[plemarquand]

Motivation:

RepositoryManager and RegistryDownloadsManager fired each delegate callback from its own unstructured Task:

    Task { await delegate?.willFetch(...) }
    ...
    Task { await delegate?.didFetch(...) }

Two independently created tasks start in whatever order the executor picks, so nothing stopped didFetch starting before willFetch. The actor proxy they awaited serializes the calls it receives, but it can't do anything about the order they arrive in.

Before #8721 these went through delegateQueue.async { ... } and every caller passed .sharedConcurrent. A concurrent queue dequeues FIFO, so delivery started in order while the callbacks still ran independently. Moving to Task dropped the ordering and didn't put anything back.

Modifications:

Add SerialEventQueue to Basics, basically an AsyncStream that a single task drains, holding the delegate it delivers to. emit yields into the stream and returns, so it doesn't block the caller, and the drain task calls the delegate one event at a time in the order they were emitted.

Both managers hold one directly now and emit through it. That replaces the two DelegateProxy types (which had turned into pass-throughs restating every protocol method).

Result:

Callbacks are delivered in the order they were emitted, so didFetch can no longer land before willFetch.

The tradeoff is that a slow delegate now delays the callbacks queued behind it, which the old .sharedConcurrent dispatch didn't do. Ordering callbacks means waiting on them, so I dont think theres a way around that short of a queue per package. These delegates format a string and write a line, so realistically it shouldn't matter.

Also stops allocating when theres no delegate. Task { await delegate?.foo() } allocated and enqueued a task even when delegate was nil, which the registry progress handler did once per chunk received.

[plemarquand]

@swift-ci test

[plemarquand]

@swift-ci test windows platform

[jakepetroules]

This Task should be saved as an ivar and cancelled on deinit. Also, it'd be ideal if you could provide a function that allows the caller for wait for the task to finish.

[plemarquand]

@swift-ci test

[plemarquand]

@swift-ci test windows

[plemarquand]

@swift-ci test windows

[plemarquand]

@swift-ci test windows platform

[plemarquand]

@swift-ci test windows platform