Define main package

[parzhitsky]

Aside from defining an entry-point package for main functionality, this PR demonstrated the process of introducing new packages in general

[parzhitsky]

PR is in draft and Pending: unclear because the convention of having the default functionality in the main package does not yet fully make sense:

• what if there are multiple "main" packages (like, in a client-server web app)?
• what if the repository is not a mono repo?