Each library in ardupilot is a folder with the source in. That avoids build
failures when there are files in libraries/. That fixes#4099 ("waf doesn't
seem to like TAGS files") and #4093 ("sim_vehicle.py /w waf /w eclipse project
broken on Cygwin").
That problem can be reproduced by just creating a file in libraries/ and trying
to build with waf.
There should be a better way to confirm a path is an ardupilot library. That
can be done later.
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