[racket] planet resolver change

I've just pushed a change to planet's module name resolver (the code
that figures out if the package is downloaded or not and, if not,
downloads and installs packages) so that it is (should be) thread
safe. This code is in that category where the ratio of importance to
testedness is not so good. I've improved it a little bit, but please
let me know if you spot any problems.

Robby