FR: Add support for lazy indexing to the Gazelle plugin

[jeffmace]

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

[jeffmace]

https://github.com/bazel-contrib/bazel-gazelle/blob/master/extend.md

[jeffmace]

I guess I should've done one last check for new PRs. @shs96c opened #397 just a few hours before I posted this.