Use Gradle lazy properties for WireExtension API

[oldergod]

Fixes #3543

Should not be breaking current callers

[JakeWharton]

Still can break people. Kotlin-based Gradle plugins will no longer be able to do = whevs.

[JakeWharton]

Suggested change

	fun permitPackageCycles() = permitPackageCycles.get()
	fun permitPackageCycles() = permitPackageCycles.set(true)

[JakeWharton]

Suggested change

	fun loadExhaustively() = loadExhaustively.get()
	fun loadExhaustively() = loadExhaustively.set(true)

[oldergod]

It was a getter before, why do we wanna change it to a setter?

[oldergod]

Stuff like this are in to not break existing consumers. Are you referring to people setting up the Wire plugin in code?

  /** For Groovy DSL assignment: `protoLibrary = true`. */
  fun setProtoLibrary(value: Boolean) {
    protoLibrary.set(value)
  }