Add a way to combine imported type stubs with local

[justhecuke]

Hey, it's pretty common for 3p code out of PIP to have incomplete or incorrect typing information.

Instead of having our local developers disable type-checking in order to get things working, I think it would be helpful to allow us to merge a local type stub (or an inline declaration of typing information near to the local usage) with the one coming from the imported module.

In this way, local developers can make progress if the upstream module is incorrect. My team, for instance, is stuck on old versions of modules for compatibility reasons and can't pick up the up-to-date correct typing information.