Improve the plugin code according to Gradle recommendations

[bsautel]

I suggested to Gradle some API improvements to fix issues I encountered when integrating lazy configuration in the plugin code in gradle/gradle#12974.

@big-guy made a kind of code review of the plugin and suggested us back some improvements we could do in the plugin code to improve it.

I learned multiple things and I think we should have a look to that. Some of this changes are breaking changes, I am wondering whether we should apply them now or wait for the future 4.0 version.