[DAEMON-146] appcd-detect: Detect engine scans paths that were already recursively scanned

Found an issue where the detect engine is finding the same thing more than once. For example, say you have the search path /opt and the ANDROID_NDK environment variable is pointing to /opt/android-ndk. The search paths will be /opt and /opt/android-ndk. It will recursively scan /opt and find the ndk in /opt/android-ndk. However it will continue to scan the /opt/android and find the same thing again. The detect engine recursively walks the search paths in series, so it should be possible to track "visited" directories or "found" items.