Fix wrong configuration being used with multiple workspace folders

[rchl]

When a workspace has multiple folders then the approach of using
one global variable to store the config breaks down as each workspace
folder can have separate configuration and the last one that gets
initialized overwrites the previous one.

Store configuration in a map looked up by workspace folder path.

[Richardk2n]

Just to make sure I understand this correctly:
There is a config file per workspace (as in pylsp workspace or project folder)
Now if you would want to lint in multiple workspaces at the same time using one instance of mypy-ls this would not work correctly?
In what usecase does this happen?

[rchl]

When you have multiple root folders in the editor then, as long as the client (editor) supports the workspaceFolders capability, those are handled as separate workspaces within a single pylsp instance.

The use case is:

• multiple root folders (workspaces) in the editor
• each folder having its own mypy.ini configuration file
• linting files within each folder should use respective mypy configuration file