[POC] Install Legal module via composer WHEN BUILDING Zikula

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | ---
| Fixed tickets | ---
| Refs tickets  | ---
| License       | MIT
| Doc PR        | ---

This is a proof of concept which shows how to install Zikula modules via composer **when building Zikula**. With this PR, the newest version of the Legal module always is installed when Zikula is built. 
The `repositories` section at the bottom is needed, because the Legal module is not yet available at Packagist.