Add support for gradle incremental annotation processing

[hotchemi]

Issue

resolves #473

Overview

This PR adds incremental compilation support. I suppose right now this feature is only for Java? 🤔

Ref

• doc: https://docs.gradle.org/nightly/userguide/java_plugin.html#sec:incremental_annotation_processing
• Dagger's issue and commit: Support incremental compilation #473 (comment)

[autonomousapps]

Kotlin does not have support for incremental annotation processing yet. There's an open issue, can't find the link...

[hotchemi]

@autonomousapps thank you I've just founded on Dagger's repo 🙇 #473 (comment)

[technoir42]

@hotchemi for incremental AP to work you also need to provide an originating element for every generated file.

From the Gradle documentation:

They must provide exactly one originating element for each file generated with the Filer API. If zero or many originating elements are provided, Gradle will recompile all source files.

For Java you can use TypeSpec.Builder.addOriginatingElement but I'm not sure what to use for Kotlin.
Here is relevant commit from Dagger: google/dagger@baf2357

[hotchemi]

@technoir42 ooh thank you let me address!