Do not attempt to reinstall python if it already exists (#394)

* Do not attempt to reinstall python if it already exists, since that python may be interpreting the current emsdk.py script that is being executed to do the installation.

* Add TODO comment about refactoring install scheme

emsdk.py

@@ -1698,6 +1698,15 @@ class Tool(object):
       print("Done installing SDK '" + str(self) + "'.")
       return True
     else:
+      # We should not force reinstallation of python if it already exists, since that very python
+      # may be interpreting the current emsdk.py script we are executing. On Windows this would
+      # lead to a failure to uncompress the python zip file as the python executable files are in use.
+      # TODO: Refactor codebase to avoid needing this kind of special case check by being more
+      # careful about reinstallation, see https://github.com/emscripten-core/emsdk/pull/394#issuecomment-559386468
+      # for a scheme that would work.
+      if self.id == 'python' and self.is_installed():
+        print("Skipped installing " + self.name + ", already installed.")
+        return True
       print("Installing tool '" + str(self) + "'..")
       url = self.download_url()