List installed plugin by package name

[thomaspatzke]

Discussed in #85

sigma plugin list shows all available plugins that can potentially be installed. sigma list targets and sigma list pipelines show what is installed, but only expose the backend/pipeline identifiers they provide, not the plugin package names.

For example, installing the elasticsearch plugin makes lucene, eql, esql, and elastalert appear in sigma list targets, but nothing maps those identifiers back to elasticsearch. So if I want to programmatically run sigma plugin uninstall elasticsearch, I have no reliable way to know it is installed in the first place, since elasticsearch does not appear as an output when running sigma list targets.