Add external reader support in pkl-gradle

[HT154]

No description provided.

[bioball]

@HT154 can we add some test coverage for this?

I think this would involve moving TestExternalReaderProcess to pkl-commons-test, and adding a corresponding test to org.pkl.gradle.EvaluatorsTest.

Hint: the Groovy syntax for maps is [key: value].
External readers would look something like:

externalModuleReaders = ["foo": new org.pkl.gradle.ExternalReader("bar")]

[netvl]

These should be marked as @Input properties, not @Nested. The @Nested annotation in Gradle is used to make Gradle process values of a map property as beans. All @Nested properties are treated as @Input, so it will work as is, but semantically values of this map are just data carriers without any annotations which do not require further processing, so I think that @Input would be more appropriate here.

[HT154]

Thanks @netvl! Do you know of any documentation that covers these subtleties? I wasn't able to find anything when I searched.

[netvl]

This is all described in Gradle docs, but it may be hard to find, especially since Gradle docs constantly evolve. Most of it comes with experience with writing plugins and sifting through tons of github tickets in the Gradle repo :)
On this particular thing, I believe, the description is here: https://docs.gradle.org/current/userguide/incremental_build.html#sec:task_input_output_annotations