Importer typehints and small importer refactor (#5611)

## Description

Hello y'all. 

One of the dev from the [beets-flask
app](https://github.com/pSpitzner/beets-flask) here. We are extending
the `ImportSession` in our application to get the interactive imports
working in our application. Overall that was not a too pleasant
experience as a ton of typehints are missing in that area of the beets
code. This PR wants to fix this ;)

### Changes

Moved importer state logic into a dataclass. Is clearer and more
isolated now in my opinion, we are not using it but I found it quite
confusing when looking at it.

Added a ton of typehints to the `importer.py` file. Predominantly the
`ImportSession`, `ImportTask` (and derivatives) and pipeline stage
decorators are now (hopefully) completely typed. Additionally I fixed
some typhint issues in the `util.__init__.py` file, mainly related to
the `PathLike` type.

If you have some strong opinions about any of the changes feel free to
revert or edit the PR.

Best,
Sebastian

P.S: I noticed that beets does not use Abstract Base Classes is there
any reason for that or is it just of historic nature? I think some Base
Classes could be adapted to use an ABC metaclass quite easily.

Author: snejus