x/tools/gopls: file watchers should be reevaluated after load

[findleyr]

Normally I would just fix this bug, but given our proximity to the v0.12.0 release I'm filing it for later:

When state changes in gopls, we re-evaluate file watchers. Currently, this is done synchronously to the state change processing, via Server.didModifyFiles->updateWatchedDirectories.

Problems with this:

• AFAIK there is no need to block change processing on updating watched directories. File watching is always going to be asynchronous, so I don't think we avoid any races by blocking while processing didChange notifications
• we should generally avoid blocking on server->client calls during notification processing, as it can lead to distributed deadlocks in certain clients
• we should awaitLoaded before re-evaluating file watches, as go list might surface new directories to watch after e.g. a go.mod change. We can't awaitLoaded during change processing.

[gopherbot]

Change https://go.dev/cl/496879 mentions this issue: gopls/internal/lsp/regtest: delete TestWatchReplaceTargets

[findleyr]

This can wait until gopls/v0.16.0, since we've been living with it for a while. But I do think this is a source of potential inconsistency.