Don't load resources for the current aircraft from several aircraft dirs

* If one has the same aircraft in several aircraft directories,
  FlightGear should not mix resources from the various aircraft
  directories. For instance, if one starts FG with:

    --fg-aircraft=/my/personal/dir:/path/to/fgaddon/Aircraft

  and one has in /my/personal/dir/ec130 a clone of the upstream
  developer repo, FlightGear should use either the upstream version from
  /my/personal/dir/ec130 or the FGAddon version from
  /path/to/fgaddon/Aircraft/ec130, but not some strange, untested hybrid
  of both.

* This commit makes sure that when the looked-up resource starts with
  Aircraft/<ac>, where <ac> is the current aircraft name [last component
  of aircraftDir = fgGetString("/sim/aircraft-dir")], then
  AircraftResourceProvider::resolve() doesn't search other aircraft
  directories if the resource isn't found under 'aircraftDir'.

* To reproduce the bug before this commit, you may add the following
  code (there is nothing specific about the SenecaII here, it's just the
  aircraft I used for testing):

    var file_path = resolvepath("Aircraft/SenecaII/flo-test");
    if (file_path != "")
      gui.popupTip("flo-test found", 2);
    else
      gui.popupTip("flo-test not found", 2);

  in a keyboard binding for the SenecaII (for instance; you may use the
  F11 binding that otherwise only prints a short message). You should
  add this to the SenecaII/SenecaII-base.xml file *that will be loaded
  by FlightGear*, let's say the one under /my/personal/dir in the
  example above (beware of the <path-cache> in autosave_X_Y.xml). Then,
  by creating or removing a file named "flo-test" in the SenecaII
  subdirectory of other aircraft dirs (for instance,
  /path/to/fgaddon/Aircraft in the example above), you can see that the
  behavior of the loaded aircraft is influenced by the contents of
  unrelated versions of the same aircraft that might be present in other
  aircraft dirs (e.g., loaded /my/personal/dir/SenecaII influenced by
  /path/to/fgaddon/Aircraft/SenecaII).

* Aircrafts loading resources using paths relative to the current
  aircraft directory (e.g., with 'resolvepath("flo-test")') are not
  affected by this kind of problem, because this scheme is handled by
  CurrentAircraftDirProvider, which does not exhibit this bug.

src/Main/globals.cxx

@@ -91,6 +91,11 @@ public:
         
         if (r.exists()) {
           return r;
+        } else {
+          // Stop here, otherwise we could end up returning a resource that
+          // belongs to an unrelated version of the same aircraft (from a
+          // different aircraft directory).
+          return SGPath();
         }
     }