Fix merge behaviour in add_pyproject_configs

**Motivation**
There's a bug in `add_pyproject_configs` where a section
```[tool.importlinter]
root_packages=["example"]
```
Is only added when there are not other sections present e.g.
```[[tool.importlinter.contracts]]
name = "Modular Design" # etc.
```

**Summary of desired enhancement**
`add_pyproject_configs` should be able to "merge" in this case, and so still add the section.
