dokka-core should use api instead implementation

[elect86]

Describe the bug

If I try to apply Dokka to a precompiled script, I get

Supertypes of the following classes cannot be resolved. Please make sure you have the required dependencies in the classpath:
    class org.jetbrains.dokka.gradle.GradleDokkaSourceSetBuilder, unresolved supertypes: org.jetbrains.dokka.DokkaConfigurationBuilder

Unless I add dokka-core as well

dependencies {
    implementation("org.jetbrains.dokka:dokka-gradle-plugin:1.4.20")
    implementation("org.jetbrains.dokka:dokka-core:1.4.20") // this
}

Expected behaviour

Working fine without dokka-core

To Reproduce

Clone this, tmp branch and try to sync

Dokka configuration

dokkaHtml {
    dokkaSourceSets.configureEach {
        sourceLink { // here the problem
            localDirectory.set(file("src/main/kotlin"))
            remoteUrl.set(URL("https://github.com/kotlin-graphics/kotlin-unsigned/tree/master/src/main/kotlin"))
            remoteLineSuffix.set("#L")
        }
    }
    finalizedBy(netlifyBadge)
}

Installation

• Operating system: macOS/Windows/Linux
• Build tool: Gradle v6.8.1
• Dokka version: 1.4.20

Additional context

Kudos to Björn for finding this out!

The problem is that the dokka-gradle-plugin here:

dokka/runners/gradle-plugin/build.gradle.kts

Line 16 in 49d4757

implementation(project(":core"))

defines dokka-core as implementation dependency, but then uses classes from dokka-core in the API, for example here:

dokka/runners/gradle-plugin/src/main/kotlin/org/jetbrains/dokka/gradle/GradleDokkaSourceSetBuilder.kt

Line 23 in 49d4757

) : DokkaConfigurationBuilder<DokkaSourceSetImpl> {

where DokkaConfigurationBuilder is used as superclass and thus is part of the API and is needed by downstream projects in their compile class path.
So it should actually declare it as api dependency, not implementation dependency.
Adding dokka-core manually to your compile class path like you did works around this bug,
but imho you should report this to the dokka project.