Fix collects for setup for single-collect package

This commit fixes single collection packages when the package has subdirectories. Previously, it would try to set up these subdirectories as collections and error. I couldn't figure out how to update the pkg tests to test this, so the PR is missing a new test.

The following setup reproduces the bug:

    $ ls foo
    bar/  info.rkt
    $ cat foo/info.rkt
    #lang setup/infotab
    (define deps '())
    (define single-collection "foo")
    $ raco pkg install foo/
    collection-path: collection not found
      collection: "bar"
      in collection directories:
       /home/asumu/plt/racket-git/add-on/5.3.4.11/collects
       /home/asumu/plt/racket-git/collects