Move real plugins out of 'fixtures' dirs (#148756)

The location of plugins was previously somewhat irrelevant, but as we
move into packages it's more important that we can find all plugins in
the repository, and we would like to be able to do that without needing
to maintain a manifest somewhere to accomplish this. In order to make
this possible we plan to find any plugin/package by spotting all
kibana.json files which are not "fixtures". This allows plugin-like code
(but not actual plugin code) to exist for testing purposes, but it must
be within some form of "fixtures" directory, and any plugin that isn't
in a fixtures directory will be automatically pulled into the system
(though test plugins, examples, etc. will still only be loaded when the
plugin's path is passed via `--plugin-path`, the system will know about
them and use that knowledge for other things).

Since this is just a rename Operations will review and merge by EOD Jan
12th unless someone has a blocking concern.

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>

test/interactive_setup_api_integration/manual_configuration_flow_without_tls.config.ts

@@ -17,7 +17,7 @@ import { services } from './services';
 export default async function ({ readConfigFile }: FtrConfigProviderContext) {
   const xPackAPITestsConfig = await readConfigFile(require.resolve('../api_integration/config'));
 
-  const testEndpointsPlugin = resolve(__dirname, './fixtures/test_endpoints');
+  const testEndpointsPlugin = resolve(__dirname, './plugins/test_endpoints');
 
   const tempKibanaYamlFile = join(getDataPath(), `interactive_setup_kibana_${Date.now()}.yml`);
   await fs.writeFile(tempKibanaYamlFile, '');