Do not add Importer again after it was initially added (#213)

Author: chrisrink10

src/basilisp/importer.py

@@ -59,6 +59,7 @@ def get_filename(self, fullname: str) -> str:
 
     def create_module(self, spec: importlib.machinery.ModuleSpec):
         mod = types.ModuleType(spec.name)
+        mod.__file__ = spec.loader_state["filename"]
         mod.__loader__ = spec.loader
         mod.__package__ = spec.parent
         mod.__spec__ = spec
@@ -103,4 +104,6 @@ def hook_imports():
 
     Once this is called, Basilisp code may be called from within Python code
     using standard `import module.submodule` syntax."""
+    if any([isinstance(o, BasilispImporter) for o in sys.meta_path]):
+        return
     sys.meta_path.insert(0, BasilispImporter())  # pylint:disable=abstract-class-instantiated

tests/basilisp/importer_test.py

@@ -0,0 +1,24 @@
+import sys
+from unittest.mock import patch
+
+import basilisp.importer as importer
+
+
+def importer_counter():
+    return sum([isinstance(o, importer.BasilispImporter) for o in sys.meta_path])
+
+
+def test_hook_imports():
+    with patch('sys.meta_path',
+               new=[]):
+        assert 0 == importer_counter()
+        importer.hook_imports()
+        assert 1 == importer_counter()
+        importer.hook_imports()
+        assert 1 == importer_counter()
+
+    with patch('sys.meta_path',
+               new=[importer.BasilispImporter()]):
+        assert 1 == importer_counter()
+        importer.hook_imports()
+        assert 1 == importer_counter()