Warn when apps folder contains things which are not applications

[bitwalker]

I've fielded a few issues from people using exrm/distillery where the issue turned out to be that there were things under apps in an umbrella project which were not actually apps (either files or folders). I addressed this by filtering the list of things in that folder based on some criteria, but since it seems like this folder is special in Mix (based on the conversation below), Mix should probably warn users when they (un)intentionally put non-application folders there. I'm not sure what the best place is though.

alisdair `** (Mix) Could not start application sdata: could not find application file: sdata.app`
alisdair that's the error. sdata doesn't show up anywhere in the code
alisdair (i get there by running `iex -S mix`)
josevalim oh... so Mix is thinking something has sdata
josevalim do you have a sdata directory in apps?
josevalim does it happen when running in the umbrella root only or in the umbrella child?