The integration test that was broken was:
```
--- FAIL: TestCreateInvalidExperiment (0.35s)
experiment_integration_test.go:192: expected a nil builder here
```
While there improve the documentation of the ExperimentSession
and see there's a method that we are not using.
This diff is a cleanup that I come up with while working
on https://github.com/ooni/probe/issues/2184.
Package github.com/ooni/probe-cli/internal/model
Shared data structures and interfaces. We include in this
package the most fundamental types. Use go doc to get
more thorough documentation about what is inside this package
and when to put a type inside this package.