Allow to react to the presence of dependencies in actions and Handlebar templates

[odrotbohm]

It would be nice if the actions and templates of user commands could be bound to the presence of a certain dependency within the project. Something similar to project.dependencies.contains("*:foo-bar:*") with the String parameter handed in effectively matching the GAV strings for dependencies.

This can already be achieved for Handlebar templates by executing a command triggering the build system and binding the result to a variable:

actions:
  - exec:
      command: ./mvnw dependency:list | grep 'jakarta.persistence-api' || echo ''
      define:
        name: jpa-present

This would allow the templates to check {{#if jpa-present}}. That said, this is currently not supported within the action files themselves. Also, running the build system for each such variable is time-consuming. It would be nice if the CLI ran such a command one, caches it and monitors the build files for changes to refresh the model.