Improve support for Playwright tests when using plugin-directory input

When the plugin doesn't live in the root of the repository (e.g.: monorepos), the Playwright workflow is not able to correctly resolve the Grafana dependency because the underlying action assumes the plugin is in the root of the repository.

The workaround is specifying the grafana-dependency as an input:

```yaml
with:
plugin-directory: packages/grafana-my-app
package-manager: npm

# The playwright action assumes that the plugin is in the root of the repository
# and gets the Grafana dependency from src/plugin.json. This is not the case for
# this plugin, so we need to explicitly set the Grafana dependency version.
run-playwright-with-grafana-dependency: ">=9.5.2"
```

We should instead find a way to automatically locate the plugin.json file in the folder specified in plugin-directory and use that instead.