Use read-pkg-up to get version number.

Previously we read the version number from `package.json` by reading from a specific file path. To do this we had to set `resolveJsonModule` to `true`.

This resulted in `lib` going from:

```
lib
 |_index.js
 |_package.json
```

to:

```
lib
 |_src
 | |_index.js
 | |_package.json
 |_package.json
```

This impacts import statements. From:

```
import { GuAutoScalingGroup } from "@guardian/cdk/lib/constructs/autoscaling";
```

to:

```
import { GuAutoScalingGroup } from "@guardian/cdk/lib/src/constructs/autoscaling";
```

Using read-pkg-up means we do not need to set `resolveJsonModule` and can keep the old `lib` structure and sane import paths.