Provide more information about source set's target / platform

[IgnatBeresnev]

It can be helpful for plugins to be able to differentiate between different targets, which are not necessarily a high-level platform - for example, Android is relevant for both the JVM and the Native (Android NDK) platforms.

It might also be useful to provide further platform distinction within a higher level platform/target. For instance, the Native target can include iOS, macOS, Linux, etc.

While at it, it'll be good to think about the naming - what should be called a target and what should be a platform, how not to confuse the two. It'd be ideal if our API was aligned with Kotlin's.

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The scope of the issue can be reduced to providing the ability to differentiate between Android and non-Android projects / source sets - this was the initial request.

[IgnatBeresnev]

Internal use cases for the Android target/platform:

• Automatically enable android-documentation-plugin
• Generate documentation for generated sources as there are code-generating tools, see Dokka showing <Error class: unknown class> for classes generated by data binding #2821