Merge pull request juju#18435 from Aflynn50/use-watcher-registry-for-…

…entity-watcher

juju#18435

The AgentEntityWatcher used the old facade.Resources approach to register its watchers. The Watch api call is going to be updated as part of other work so this is getting the facade ready for that.

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<!-- Why this change is needed and what it does. -->

## Checklist

<!-- If an item is not applicable, use `~strikethrough~`. -->

- [x] Code style: imports ordered, good names, simple structure, etc
- [x] Comments saying why design decisions were made
- [x] Go unit tests, with comments saying what you're testing
- [x] [doc.go](https://discourse.charmhub.io/t/readme-in-packages/451) added or updated in changed packages

## QA steps
Run the unit tests and smoke.
<!-- 

Describe steps to verify that the change works. 

If you're changing any of the facades, you need to ensure that you've tested
a model migration from 3.6 to 4.0 and from 4.0 to 4.0.

The following steps are a good starting point:

 1. Bootstrap a 3.6 controller and deploy a charm.

```sh
$ juju bootstrap lxd src36
$ juju add-model moveme1
$ juju deploy juju-qa-test
```

 2. Bootstrap a 4.0 controller with the changes and migrate the model.

```sh
$ juju bootstrap lxd dst40
$ juju migrate src36:moveme1 dst40
$ juju add-unit juju-qa-test
```

 3. Verify no errors exist in the model logs for the agents. If there are
 errors, this is a bug and should be fixed before merging. The fix can
 either be applied to the 4.0 branch (preferable) or the 3.6 branch, though
 that needs to be discussed with the team.

```sh
$ juju debug-log -m dst40:controller
$ juju debug-log -m dst40:moveme1
```

 4. We also need to test a model migration from 4.0 to 4.0.

```sh
$ juju bootstrap lxd src40
$ juju add-model moveme2
$ juju deploy juju-qa-test
```

```sh
$ juju migrate src40:moveme2 dst40
$ juju add-unit juju-qa-test
```

 5. Verify that there are no errors in the controller or model logs.

```sh
$ juju debug-log -m dst40:controller
$ juju debug-log -m dst40:moveme2
```

--
## Links

<!-- Link to all relevant specification, documentation, bug, issue or JIRA card. -->

**Jira card:** [JUJU-7104](https://warthogs.atlassian.net/browse/JUJU-7104)



[JUJU-7104]: https://warthogs.atlassian.net/browse/JUJU-7104?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

apiserver/common/leadership.go

@@ -17,8 +17,6 @@ import (
 	"github.com/juju/juju/state"
 )
 
-//go:generate go run go.uber.org/mock/mockgen -typed -package mocks -destination mocks/leadership.go github.com/juju/juju/apiserver/common LeadershipPinningBackend,LeadershipMachine
-
 // LeadershipMachine is an indirection for state.machine.
 type LeadershipMachine interface {
 	ApplicationNames() ([]string, error)