refactor: interfaces and data types into the model package (#642)

## 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

internal/engine/model/logger.go

@@ -1,40 +0,0 @@
-package model
-
-// Logger defines the common interface that a logger should have. It is
-// out of the box compatible with `log.Log` in `apex/log`.
-type Logger interface {
-	// Debug emits a debug message.
-	Debug(msg string)
-
-	// Debugf formats and emits a debug message.
-	Debugf(format string, v ...interface{})
-
-	// Info emits an informational message.
-	Info(msg string)
-
-	// Infof formats and emits an informational message.
-	Infof(format string, v ...interface{})
-
-	// Warn emits a warning message.
-	Warn(msg string)
-
-	// Warnf formats and emits a warning message.
-	Warnf(format string, v ...interface{})
-}
-
-// DiscardLogger is a logger that discards its input
-var DiscardLogger Logger = logDiscarder{}
-
-type logDiscarder struct{}
-
-func (logDiscarder) Debug(msg string) {}
-
-func (logDiscarder) Debugf(format string, v ...interface{}) {}
-
-func (logDiscarder) Info(msg string) {}
-
-func (logDiscarder) Infof(format string, v ...interface{}) {}
-
-func (logDiscarder) Warn(msg string) {}
-
-func (logDiscarder) Warnf(format string, v ...interface{}) {}