Explore the possibility of making the plugin an ordinary module

[qurbonzoda]

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On current moment, plugin is not in classpath of integration module, because plugin is included build (to use current version of plugin in examples). We can add it to classpath in some way, but now kotlinx.benchmark.gradle.BenchmarkConfiguration depends on BenchmarksExtension, looks like we need to remove this dependency to reuse it.

I have a question, why is included build is used at all? Maybe it's possible to make it as an ordinary module, and after that integration module will be moved just to integrationTest source set under it (the same way as it's done in kotlinx-atomicfu and binary-compatibility-validator gradle plugins)

Originally posted by @whyoleg in https://github.com/Kotlin/kotlinx-benchmark/pull/37#discussion_r623687850