Make TidyImports + notebook integration idemponent

Order of imports was not correct on the first click of the TidyImports
button. This PR fixes that issue.

lib/python/pyflyby/_comms.py

@@ -121,24 +121,23 @@ def _reformat_helper(input_code, imports):
 
 def extract_import_statements(text):
     """This is a util for notebook interactions and extracts import statements
-    from some python code.
-
+    from some python code. This function also re-orders imports.
     Args:
         code (str): The code from which import statements have to be extracted
 
     Returns:
-        (list[str], str): The first returned value is a list of all import
-        statements. The second returned value is the remaining code after
+        (str, str): The first returned value contains all the import statements.
+        The second returned value is the remaining code after
         extracting the import statements.
     """
     transformer = SourceToSourceFileImportsTransformation(text)
-    imports = [str(im.pretty_print()) for im in transformer.import_blocks]
+    imports = '\n'.join([str(im.pretty_print()) for im in transformer.import_blocks])
     remaining_code = "\n".join([str(st.pretty_print()) if not isinstance(st, SourceToSourceImportBlockTransformation) else "" for st in transformer.blocks])
     return imports, remaining_code
 
-def collect_code_with_imports_on_top(imports, cell_array):
-   return (
-        "\n".join(imports)
+def collect_code_with_imports_on_top(imports: str, cell_array):
+    return (
+        imports
         + "\n"
         + "\n".join(
             [
@@ -185,7 +184,7 @@ def _recv(msg):
         elif data["type"] == TIDY_IMPORTS:
             checksum = data.get("checksum", '')
             cell_array = data.get("cellArray", [])
-            import_statements, processed_cell_array = [], []
+            import_statements, processed_cell_array = "", []
             for cell in cell_array:
                 text = cell.get("text")
                 cell_type = cell.get("type")
@@ -201,6 +200,6 @@ def _recv(msg):
                     "checksum": checksum,
                     "type": TIDY_IMPORTS,
                     "cells": processed_cell_array,
-                    "imports": [im.strip() for im in import_statements],
+                    "imports": import_statements,
                 }
             )