Optional plugin dependencies

[lefou]

It would be nice to have optional plugin dependencies.

Use Case:
The new Source Scheme Plugin (https://github.com/SBuild-org/sbuild-sourcescheme-plugin) is capable of providing source jars for arbitrary dependencies. It registeres a "source:" scheme which will be used e.g. by the eclipse plugin to download source jars.

If a user uses e.g. the Aether plugin to provide transitive dependencies with Eclipse Aether (https://github.com/SBuild-org/sbuild-aether-plugin), it would be great, if the Aether plugin would reconfigure the Source Scheme plugin to also support "aether:" dependencies. But, it should also work, if no Source Scheme plugin is present.

What is needed?

• A way to express optional dependencies. The PluginDependency traits that is used by the PluginWithDependencies trait need another implementation supporting optional plugins.
• A way to check if a plugin is available. The Plugin object need a new method: def isAvailable(pluginClass: String, version: String): Boolean, whereas the version string might even be a range.
• Some nice API to only process a plugin, if it is available. I think of a method expecting the name and version of the required plugin and a closure, which requires the available plugin. Of course, the closure will only be loaded, if the plugin is available, avoiding ClassNotFoundErrors and the like.