Simplify creations of separate benchmark source sets in multiplatform projects

[fzhinkin]

Documentation describes a way to place benchmarks in separate source sets. It kinda works when you need a single source set associated with a particular target. However, creating a separate common benchmark source set where benchmarks could be executed for each and every target is quite difficult.

It would be nice to provide an extension function for Project, that'll do that itself, instead of delegating this tedious work to users.

[fzhinkin]

This is how "manual" configuration looks like now:

kotlin {
    sourceSets {
        val benchmarkMain by creating {
            dependencies {
                implementation("org.jetbrains.kotlinx:kotlinx-benchmark-runtime:0.4.13")
            }
        }
    }

    val bmm = sourceSets.getByName("benchmarkMain")

    targets.matching { it.name != "metadata" }.all {
        compilations.create("benchmark") {
            associateWith(this@all.compilations.getByName("main"))
            defaultSourceSet {
                dependencies {
                    implementation("org.jetbrains.kotlinx:kotlinx-benchmark-runtime:0.4.13")
                }
                dependsOn(bmm)
            }
        }
    }

    targets.matching { it.name != "metadata" }.all {
        benchmark.targets.register("${name}Benchmark")
    }
}

Yes, it could be cleaned up and simplified. Yet, it's hard to come up with something like that unless you spend enough time writing and debugging build scripts.