KT-71784 Fix classpath for KMP shared source sets (#3942)

DGP fetches information from KGP to automatically setup the DokkaSourceSets. In KMP projects DGP incorrectly aggregated the classpath for 'parent' source sets (e.g. commonMain) from all children. This sometimes worked, but sometimes failed.

For each KotlinSourceSet (KSS) DGP needs to use _all_ compilations to determine whether they are 'publishable' ('test' source sets are not publishable by default), and the Kotlin target (for example. 

DGP needs to use the specific compilations for a KSS to get the exact classpath for Dokka.

This PR makes DGP track all and specific compilations separately.

Fix [KT-71784](https://youtrack.jetbrains.com/issue/KT-71784)

Fix #3860

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Co-authored-by: Oleg Yukhnevich <whyoleg@gmail.com>

Author: adam-enko