x/tools/gopls: retry workspace initialization as needed

[stamblerre]

We currently try loading a user's workspace with ./... on gopls start, and if that fails, we mark it as an error and don't try again. We are able to reclaim some workspace packages, but we don't retry the full workspace load. We should determine a strategy for handling this.

[stamblerre]

We currently retry the initial workspace load if the go.mod file changes. This seems like a good starting point, and we can consider other cases as issues are reported. This is too general of an issue to be useful any longer, so closing.