Local plugins not found in alternative snap dir

[Claudio Matsuoka]

If an alternative location such as build-aux/snap is used to store the snapcraft yaml file, local plugins fail to load from build-aux/snap/plugins. Instead, snapcraft keeps loading them from the standard snap directory location (snap/plugins).

To reproduce, create a project using a local plugin and use build-aux/snap instead of snap to store the yaml files and the local plugins directory. The build will fail with:
```
Using 'build-aux/snap/snapcraft.yaml': Project assets will be searched for from the 'build-aux/snap' directory.
Searching for local plugin for local
Failed to load plugin: unknown plugin: 'local'
```

[Sergio Schvezov (sergiusens)]

https://github.com/snapcore/snapcraft/pull/2551