Put flake8 behind LSP

[karthiknadig]

See what it will take to put the linters behind LSP. As a part of the experiment put flake8 behind LSP.

[karthiknadig]

Did some initial investigation and looks like we have to discuss some UX when it comes to getting linters on user machine. Things to discuss:

1. Currently we provide a command to set linters. Upon selecting a linter we install the said linter into the user's selected python environment. With the new approach we don't have to install linter into user's environment. We have it already installed in the linting extension. We may have to point the user to installed linting extension. The command itself still has purpose in providing more details, like it could show the already installed linting extensions and allow users to choose from that.
2. Respecting existing settings, we have several settings specific to linters and we need to decide which ones we want to support.
   • python.linting.enabled: Global switch for linters. Can be used to turn off all linters.
   • python.linting.<linter>Enabled: Switch to turn on/off individual linters.
   • python.linting.<linter>Path: Path to specific linting binary. With the new linting extension, this can be a path to binary (in which case we will attempt to find the importable library), path to dir containing the importable binary, or <linter> in which case it will wither use the linter installed in the environment, or use the one shipped with the extension.
   • python.linting.<linter>Args: Additional arguments to linters.
   • python.linting.<linter>CategorySeverity: Translation between linter severity and VS Code's severity.
3. We may need to provide a way for users to control when they get linting, onType, onSave, or manual. We need to decide if this is enough or we need more. For manual we can use the existing Run Linter command to do linting on demand.
4. Logging for linting via LSP, and linters will have their own Output panel.