Message `Linter xxx not installed` displayed twice

[DonJayamanne]

• Create a workspace

• Create new virtual environment

• Create a python file

• You'll get a prompt to install pylint, dismiss that prmopt

• Disable pylint from settings.json and save the settings

• Enable pylint from settings.json and save the settings

• You'll get two prompts (one after the other) with the message Linter xxx not installed

@MikhailArkhipov I believe this is something you identified a few days ago.

[MikhailArkhipov]

Sometimes linting is executed twice: once for interpreter change event and another for settings change event. We may want to look if we can handle this better.