Historically, extensions have always been initialized one at a time.

Each time an extension is enabled, a number of things have to be

registered in Django, and some of this involves rebuilding some internal

state, which is not especially cheap.
This change helps to batch this work during load so that we cut back on

how often we perform these operations. A batch is processed through 3

steps:


Start the initialization of each extension in the batch. This is all

   the work done before registering apps in the Django app registry.


Register all apps across all extensions in the batch in one update.


Finish initialization of all extensions in the batch.


If there are any issues registering the list of apps, the extensions

will be broken out of the batch and initialized one-by-one. This helps

to isolate any broken extensions and allow the rest to initialize and

register.
The extension manager will batch lists of extensions together that have

the same dependency/requirement fulfillment status. so all extensions

that do not depend on another extension will be in one batch, followed

by all extensions that depend on those extensions, and so on.
As part of this, a bug with extension dependencies and initialization

was fixed. If an extension cannot be initialized, then none of the

extensions that depend on it (directly or indirectly) can either. All

will have load errors stored, giving admins details on what went wrong.
Note that the order of initialization within batching does mean that if

extension A tries to reach into extension B during initialize(), it

may no longer be able to do that without problems unless it lists B as a

requirement. It couldn't safely do this before either, since extension

order was never guaranteed or in any way stable. With the

requirement-based batches, there's at least now a stable way of doing

this if needed. In practice, I'm not aware of any extensions that do

this.

Unit tests pass.
Smoke-tested with a bunch of extensions. Tried enabling and disabling,

doing full reloads, scans. No problems.