Fixes processing dependencies like (:feature :windows "winhttp")

This fixes issue svspire#1.

The problem was because `asdf::resolve-dependency-spec` can return nil if feature is not supported in the current lisp implementation:

```lisp
CL-USER> (member :windows *features*)
NIL

CL-USER> (asdf::resolve-dependency-spec :dexador '(:feature :windows "winhttp"))
NIL

;; but
CL-USER> (asdf::resolve-dependency-spec :dexador "winhttp")
#<ASDF/SYSTEM:SYSTEM "winhttp">
```

quickfork.lisp

@@ -103,9 +103,12 @@ type (e.g. :git, :mercurial, etc.) and their upstream repository location")
 
 (defun direct-dependencies (project-name)
   "Return the direct dependencies of project"
-  (mapcar (lambda (dependency)
-            (asdf::resolve-dependency-spec project-name dependency))
-          (asdf:system-depends-on (asdf:find-system project-name))))
+  (loop with main-system = (asdf:find-system project-name)
+        with dependencies = (asdf:system-depends-on main-system)
+        for dependency in dependencies
+        for dep-system = (asdf::resolve-dependency-spec project-name dependency)
+        when dep-system
+          collect dep-system))
 
 (defun %all-dependencies (project-list accum-table)
   (when project-list