Reduce the amount of initial work when walking a workspace

[dbanck]

• Part of Improve language extensibility #1614

---

When the language server starts, we walk the whole workspace, checking each folder we encounter.

Instead of scheduling all jobs as soon as we encounter a directory, we could check if the directory contains any files relevant to the language server (.tf, .tf.json, .tfvars, .tfvars.json, …) and take a note of if by scheduling a discovery job. In the new discovery job, we create the necessary state entities and can decide if it makes sense to schedule further jobs. Or create small state entries (just Path) during walking and do everything else on open.

[dbanck]

With this change, initialize for a large workspace now takes 0.1s instead of 17.63s on my machine.