refactor(ivy): make return value of define(Component|Directive|Pipe|Injector|Injectable) private (#23371)

Ivy definition looks something like this:

```
class MyService {
  static ngInjectableDef = defineInjectable({
    …
  });
}
```

Here the argument to `defineInjectable` is well known public contract which needs
to be honored in backward compatible way between versions. The type of the
return value of `defineInjectable` on the other hand is private and can change
shape drastically between versions without effecting backwards compatibility of
libraries publish to NPM. To our users it is effectively an `OpaqueToken`.

By prefixing the type with `ɵ` we are communicating the the outside world that
the value is not public API and is subject to change without backward compatibility.

PR Close #23371

packages/core/src/metadata/ng_module.ts

@@ -6,13 +6,14 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {InjectorDef, InjectorType, defineInjector} from '../di/defs';
+import {InjectorType, defineInjector, ɵInjectorDef} from '../di/defs';
 import {convertInjectableProviderToFactory} from '../di/injectable';
 import {Provider} from '../di/provider';
 import {Type} from '../type';
 import {TypeDecorator, makeDecorator} from '../util/decorators';
 
 
+
 /**
  * A wrapper around a module that also includes the providers.
  *