FR: Add support for lazy indexing to the Gazelle plugin

I would like to have this functionality but don't want to duplicate effort if work has already been started by someone else.

**Proposal**

```
# gazelle:java_search components/a com.example.a
# gazelle:java_search components/b com.example.b
```

Start by locating the Gazelle plugin code that handles the `gazelle:java_search` directives and its indexing behavior. Compare the proposed component and package directives with the current search flow; done means the plugin supports lazy indexing without duplicating existing work.

Written by the indexing model from the issue text.