Bugfix for MESHROOM_SUBMITTERS_PATH env variable

Setting MESHROOM_SUBMITTERS_PATH caused meshroom to try to load the bundled
simpleFarmSubmitter plugin twice, ignoring the path in the variable.

I believe the problem lies with the way loadPlugins() works in meshroom.core,
because it's trying to import the package name "submitters" more than once.  The
second time, import_module() just returns a reference to the

This is a revised PR based on feedback from the first one I submitted last year,
which went stale and then got erased as I was stumbling my way through git.

meshroom/core/__init__.py

@@ -260,10 +260,7 @@ def loadSubmitters(folder, packageName):
 # - Nodes
 loadAllNodes(folder=os.path.join(meshroomFolder, 'nodes'))
 # - Submitters
-subs = loadSubmitters(meshroomFolder, 'submitters')
-# -  additional 3rd party submitters
-if "MESHROOM_SUBMITTERS_PATH" in os.environ:
-    subs += loadSubmitters(os.environ["MESHROOM_SUBMITTERS_PATH"], 'submitters')
+subs = loadSubmitters(os.environ.get("MESHROOM_SUBMITTERS_PATH", meshroomFolder), 'submitters')
 
 for sub in subs:
     registerSubmitter(sub())