Use plural form default output directory 'generated/sources'

[holgerstolzenberg]

Currently the plugin per default generates the generated java code into "$project/$buildDir/generated/source".
This kinda breaks Gradle best practices, please see screenshot below:

As you can see, all other stuff in the build directory uses a plural based syntax. Even the protobuf plugin does that e.g. for "extracted-protoS" and so on. Also for that manner the generated code is not aggregated with the other generated sources in their respective sourceSets under "generated/sourceS".

Please align the plugins output to the best practices.

Optionally:
What is the reasoning behind not having the configurable?