Dependency Injection: make DI micro-framework a standalone package

[lhaze]

Dependency Injection micro-framework is deserving of its own package. Make preparations to separate it.

Features that are not done yet:

• [ ] dependencies of DI (pca.utils.errors) are made standalone first, #83
• [ ] support the contextualization of the scope, #78
• [ ] scope: thread, #9
• [ ] scope: request, #9
• [ ] scope: session, #9
• [ ] inheritance support while requesting by interface, #20
• [ ] generics support while requesting by interface
• [ ] prepare quickstart guide
• [ ] prepare API docs
• [ ] make a standalone repo
• [ ] migrate commits & issues to the new repo