Technically, it's two changes:

1. Changing `snippets` to `partialsCallback` to reduce the amount of upfront IO/memory.
2. Changing the mocked-up filename to start with `~/theAngModule/`. So modules implemented in
   afform look more look other modules. Makes it work with `cv ang:html:list` and `cv ang:html:show`.

ng-include was failing with ~afform* but works with ~/afform*

Improve efficiency by not fetching layout if not needed in api.get
Fix test failure by correctly merging existing metadata during update

Fields should be allowed to be inside other tags as long as they're
contained in an af-fieldset.

The af-fieldset directive was ambiguiously set to restrict: "AE"
but only "E" was supported. I want to pick one and stick with it, and
I prefer "A" because the fieldset is also a container, so div or fieldset
seem like the appropriate tag names.

PART A: EXPORT LIST

Suppose you create a form `downstreamForm.aff.html`.

The metadata for `downstreamForm` will indicate that it exports an element `<downstream-form/>`:

```php
$angularModules['downstreamForm']['exports']['el'] = ['downstream-form'];
```

PART B: AUTO-REQUIRES

Suppose that:

1. he form `downstreamForm.aff.html` uses element `<upstream-el/>`.
2. The `$angularModules['upsteamMod']['exports']['el']` defines an exported element `upstream-el`.

Then the `downstreamForm` module will automatically requires the `upstreamMod`, as in

```php
$angularModules['downstreamForm']['requires'][] = 'upstreamMod';
```

Temporarily rename <af-model-prop> to <af-will-be-model> (so we can rename existing <af-model> easily)

__Before__: Scan each `$extension` for `$extension/afform/*/meta.json`

__After__: Scan each `$extension` for `$extension/ang/*.aff.json`

__Comments__: This is a significant breaking change for v0.2.

It preserves some key design-goals from the old naming convention

* The symbol in HTML (e.g.  `afform-email`) should have symmetry with the file-name (e.g.  `afform/Email/*` or `afform-email.*`)
* The file-name in the base-code provided by an extension should match the file-name in the local override folder.

However, the new naming convention also:

* Makes it easier downstream to choose their own prefixes. We're not boxed-in or hard-coded to `afform-`.
  This will be useful, e.g., with distinguishing the stdlib from the business-forms that use the stdlib.
* Puts Afform-Angular content in the same folder structure as Regular-Angular content.