Problem: We're very unfriendly when you point at a non-package directory

[clacke]

$ ./racket2nix --thin /tmp
extract-pkg-dependencies: contract violation
  expected: a procedure
  given: #f
  in: the 1st argument of
      (->*
       ((-> symbol? (-> any/c) any/c))
       (#:build-deps?
        boolean?
        #:filter?
        boolean?
        #:versions?
        boolean?)
       (listof
        (or/c
         string?
         (cons/c string? (listof any/c)))))
  contract from: <collects>/pkg/lib.rkt
  blaming: (/home/clacke/git/fractalide/racket2nix/nix/racket2nix.rkt main)
   (assuming the contract is correct)
  at: <collects>/pkg/lib.rkt:270.3
  context...:
   /nix/store/di03ll58yqr8bg2c47j70pgnrx2i5qwy-racket-7.0/share/racket/collects/racket/contract/private/blame.rkt:347:0: raise-blame-error16
   /nix/store/di03ll58yqr8bg2c47j70pgnrx2i5qwy-racket-7.0/share/racket/collects/racket/contract/private/arrow-val-first.rkt:431:5: mk-call
   /nix/store/di03ll58yqr8bg2c47j70pgnrx2i5qwy-racket-7.0/share/racket/collects/racket/contract/private/arrow-val-first.rkt:428:3
   /home/clacke/git/fractalide/racket2nix/nix/racket2nix.rkt:786:29
   (submod "/home/clacke/git/fractalide/racket2nix/nix/racket2nix.rkt" main): [running body]
   for-loop
   run-module-instance!125

(not isolated to the --thin mode, this has been there from day 0)

Solution: When (get-info/full) returns #f, raise a user error explaining that there's no valid info.rkt.