Fetch transitive dependency sources into Playground Books

Arena currently fetches the sources of direct dependencies and integrates them (flattened) into `UserModule`. It _does not_ fetch sources of transitive dependencies, i.e. dependencies that are dependencies of your primary dependency.

The sources are actually there, in `.build/checkouts`, just need to pull them in.

Example

```
arena https://github.com/finestructure/ArenaTest --book
```

```
$ ls -l SPM-Playground/SPM-Playground.playgroundbook/Contents/UserModules/UserModule.playgroundmodule/Sources/
total 8
-r--r--r-- 1 sas staff 52 2 Mar 09:18 ArenaTest-ArenaTest.swift
```