[plugin-helpers] Fix dependency resolution (#104556) (#104721)

This builds on the changes introduced in #104512, fixing dependency
resolution for the plugin-helpers CLI.  Instead of building a plugin in
isolation, this performs plugin discovery and filters the optimizer
lisst down to the requested plugin.

Testing
1) `cd examples/bfetch_explorer`
1) `node ../../scripts/plugin_helpers build --kibana-version=8.0.0-SNAPSHOT`

Co-authored-by: Jonathan Budzenski <jon@budzenski.me>

Author: kibanamachine

packages/kbn-plugin-helpers/src/tasks/optimize.ts

@@ -31,7 +31,7 @@ export async function optimize({ log, plugin, sourceDir, buildDir }: BuildContex
     pluginPaths: [sourceDir],
     cache: false,
     dist: true,
-    pluginScanDirs: [],
+    filter: [plugin.manifest.id],
   });
 
   const target = Path.resolve(sourceDir, 'target');