input: make imported plugin modules known

We must make the imported plugin modules visible under their mangled
name. Otherwise the pickling of projects may fail because the name is
unknown to the import machinery.

Fixes #459.

pym/bob/input.py

@@ -3309,6 +3309,7 @@ def __loadPlugin(self, mangledName, fileName, name):
             spec = spec_from_loader(mangledName, loader)
             mod = module_from_spec(spec)
             loader.exec_module(mod)
+            sys.modules[spec.name] = mod
         except SyntaxError as e:
             import traceback
             raise ParseError("Error loading plugin "+fileName+": "+str(e),