[racket-dev] Planet 2 Beta Release

I've pushed a few changes in consultation with Jay. 

The changes are small, so far, but the most prominent change is that
the interpretation of a package source is now determined syntactically.
If you want to install from a local directory "fish-tank", you need to
write

 raco pkg install fish-tank/

or

 raco pkg install --type dir fish-tank

because

 raco pkg install fish-tank

will always contact the package name service to try to resolve "fish-tank".