Can't use proguard.cfg file name is used as appendCfgFile in build.gradle

[kisg]

proguard.cfg filename has a special meaning in our Gradle plugin, it replaces the whole Gradle configuration with its content.

While this behavior is documented, it can get confusing if someone is using this filename in build.gradle to append the configuration:

    moeMainProGuard {
        appendCfgFile "proguard.cfg"
    }

The Gradle plugin should handle this special case gracefully (e.g. throwing an error, or changing the behavior to append only). We should also document this special case in the Gradle docs.