EZP-21562: Create synchronized repositories for API & SPI

http://jira.ez.no/browse/EZP-21562

Adds the required composer.json files to `eZ/Publish/API` and `eZ/Publish/SPI` in order to create the split repositories for those subtrees: ezpublish-api and ezpublish-spi.

The subtree splitting is done in a crontab with this script:

### ezpublish-api
```bash
git clone git@github.com:ezsystems/ezpublish-kernel.git
git remote add ezpublish-api git@github.com:ezsystems/ezpublish-kernel-api.git
git filter-branch --prune-empty --subdirectory-filter eZ/Publish/API master
git push ezpublish-api master
```

### ezpublish-spi
```bash
git clone git@github.com:ezsystems/ezpublish-kernel.git
git remote add ezpublish-api git@github.com:ezsystems/ezpublish-kernel-spi.git
git filter-branch --prune-empty --subdirectory-filter eZ/Publish/SPI master
git push ezpublish-spi master
```

## TODO
- [ ] Add dedicated README.md to the subfolders
- [ ] Create the repositories & setup the scripts