buildDir is not lazily evaluated for determining the generated files base directory

If I change the `buildDir` in a gradle project to a non-default location, the plugin (in version 0.9.2) does not consider the changed `buildDir` for the output directory of the generated source code if it has already been initialized. Workaround is to initialize the plugin after changing the buildDir using apply:
```
buildDir = ....
apply(plugin = "com.google.protobuf")
```
The plugin should be able to lazily detect changes to the buildDir so that I can declare it in the plugins block the usual way.