Investigate startup performance when parsing sourcemap file

[JohnnyMorganz]

On startup in my example codebase, it takes 11s to call WorkspaceFolder::setupWithConfiguration, where 8s is the cost to update the sourcemap. We will pay this sourcemap cost whenever the sourcemap changes. We should see if there are any performance gains from sourcemap parsing.

[JohnnyMorganz]

Seems to mainly come from writePathsToMap