Original `ComponentRegistrar` exposes Project to its registration method,
so plugins should manually register extensions to it. To prepare for
possible unbound compiler from Project API in future new `K2ComponentRegistrar`
introduced which provides registration method without Project at all
This scheme will be common for all compiler plugins with K1 and K2 support:
- `plugin-common` contains classes shared with K1 and K2 implementations (if any)
- `plugin-k1` contains implementation for K1 compiler
- `plugin-k2` contains implementation for K2 compiler
- `plugin-backend` contains implementation for backend extensions (if any)
- `plugin-cli` is module for registration of plugin in CLI compiler
- `plugin` is a root module with tests and all submodules embedded
This structure is needed to distinguish parts related to different frontends,
which is needed for proper dependencies settings for Kotlin IDE plugins
Dmitriy Novozhilov