Cannot set yaml.schemas if using multi-root workspaces

Possible regression of #50

I'm using:

$ code --version
1.38.1
b37e54c98e1a74ba89e03073e5a3761284e3ffb0
x64

and VSCode-YAML version:

0.5.3

I have configured a multi-root workspace:

In the folder .vscode/settings.json, I have the following:

{
  "yaml.schemas": {
    "./solution/packages/step-templates/lib/schema/step-template-schema.json": "./solution/packages/step-templates/lib/steps/**/*.yaml"
  }
}

The setting is dimmed and hovering over it produces the message:

This setting cannot be applied now. It will be applied when you open this folder directly.

Reproduce the issue using the multi-root workspace and the .vscode/settings.json configuration shown, focusing on the yaml.schemas setting and its dimmed state. Trace the extension's handling of workspace settings; done means the schema mapping can be applied in a multi-root workspace without requiring the folder to be opened directly.