CB-7124: Fix `cordova platform add <platform>@version` with `--usegit`

https://issues.apache.org/jira/browse/CB-7124

The `cordova_git` function didn't consider that platform strings can contain `@version` annotations. I refactored the code a bit to keep the `@version` check in a single place, namely in the `Platform` object.

Please let me know if this patch is acceptable in this form.