Make Scala plugin optional dependency

[OlliKiljunen]

We are planning to move REPL into a separate plugin (see #484). After that, the only thing that keep our plugin dependent on Scala plugin is ScalaSDK class.

ScalaSDK class gets loaded only if user installs a module which has a scala-sdk as an external library. Ideally, unless that happens, our plug-in should be usable without Scala plugin.

To achieve this

• make scala plugin an optional dependency
• in CommonLibraryProvider::createLibrary check that Scala plugin is loaded before instantiating ScalaSDK class.

It is not sure if this is possible, so investigate.

Blocked by #484.

[OlliKiljunen]

Decision: Let's keep this ticket open for now to remind us of the possibility to some day make the plugin support other languages as well.