pyimport-insert-missing: Add support for grepping in whole project

[twmr]

First of all thank you very much for creating this project! It is another project of yours that I want to use in my emacs setup.

Currently pyimport-insert-missing only searches for imports in the open python-mode buffers. Given that there a projects like e.g. deadgrep and dumbjumb that search for strings in the whole project, I think that it makes that pyimport also supports whole project grepping.

[Wilfred]

I'm not using pyimport much these days, but I'd be willing to take PRs :)