importer: fix glaring bug in find_related()

Overwriting 'fullname' variable caused basically nonsensical filtering.
Result was including the module being searched in the list of
dependencies, which was causing ModuleResponder to send it early, which
was causing contexts to start importing the module before preloading of
dependencies had completed.

mitogen/master.py

@@ -401,7 +401,9 @@ class ModuleFinder(object):
             if level == -1:
                 modnames = [modname, '%s.%s' % (fullname, modname)]
             else:
-                modnames = [self.resolve_relpath(fullname, level) + modname]
+                modnames = [
+                    '%s.%s' % (self.resolve_relpath(fullname, level), modname)
+                ]
 
             maybe_names.extend(modnames)
             maybe_names.extend(
@@ -425,11 +427,12 @@ class ModuleFinder(object):
         found = set()
 
         while stack:
-            fullname = stack.pop(0)
-            fullnames = self.find_related_imports(fullname)
-            stack.extend(set(fullnames).difference(found, stack, [fullname]))
-            found.update(fullnames)
+            name = stack.pop(0)
+            names = self.find_related_imports(name)
+            stack.extend(set(names).difference(found, stack))
+            found.update(names)
 
+        found.discard(fullname)
         return sorted(found)