## Checklist
- [x] I have read the [contribution guidelines](https://github.com/ooni/probe-cli/blob/master/CONTRIBUTING.md)
- [x] reference issue for this pull request: https://github.com/ooni/probe/issues/1885
- [x] related ooni/spec pull request: N/A
Location of the issue tracker: https://github.com/ooni/probe
## Description
This PR contains a set of changes to move important interfaces and data types into the `./internal/model` package.
The criteria for including an interface or data type in here is roughly that the type should be important and used by several packages. We are especially interested to move more interfaces here to increase modularity.
An additional side effect is that, by reading this package, one should be able to understand more quickly how different parts of the codebase interact with each other.
This is what I want to move in `internal/model`:
- [x] most important interfaces from `internal/netxlite`
- [x] everything that was previously part of `internal/engine/model`
- [x] mocks from `internal/netxlite/mocks` should also be moved in here as a subpackage
Each tutorial provides you a piece of knowledge. Add some text in
the main readme file explaining the big picture to the reader.
Closes https://github.com/ooni/probe/issues/1819
This change should simplify the pipeline's job.
Reference issue: https://github.com/ooni/probe/issues/1817.
I previously dismissed this possibility, but now it seems clear it
is simpler to have a very tabular data format internally and to
convert such a format to OONI's data format when serializing.
The OONI data format is what the pipeline expects, but processing
is easier with a more linear/tabular format.
