sbt: allow irregular plugins to be configured

Sometimes plugins doesn't follow the form

```
addSbtPlugin("${plugin.org}" % "${plugin.artifact}" % "${plugin.version}")
```

for instance

```
addDependencyTreePlugin
```

This commit allow free form plugin dependency.

tests/modules/programs/sbt/plugins.nix

@@ -15,12 +15,14 @@ let
   };
 
   plugins = [ dependencyGraph projectGraph ];
+  pluginsExtra = [ "addDependencyTreePlugin" ];
 
   pluginsSbtPath = ".sbt/1.0/plugins/plugins.sbt";
 
   expectedPluginsSbt = pkgs.writeText "plugins.sbt" ''
     addSbtPlugin("net.virtual-void" % "sbt-dependency-graph" % "0.10.0-RC1")
     addSbtPlugin("com.dwijnand" % "sbt-project-graph" % "0.4.0")
+    addDependencyTreePlugin
   '';
 
 in {
@@ -28,6 +30,7 @@ in {
     programs.sbt = {
       enable = true;
       plugins = plugins;
+      pluginsExtra = pluginsExtra;
       package = pkgs.writeScriptBin "sbt" "";
     };