x/tools/gopls: determine how frequently to reload packages with missing dependencies

[stamblerre]

Currently, when a user's package has a dependency that we can't resolve, we try to reload the package on every key stroke. We take care not to re-check it unless the metadata has changed, but it's still expensive to call packages.Load on every keystroke.

I wonder if we might be able to somehow make use of the goimports scanning logic in gopls to do this in a better way. It's possible that we can't do better in GOPATH mode (or in another build system), but I'd think we might be able to do something better in module mode.

@heschik: Is this a reasonable idea, or should we just stick with the current approach?

[gopherbot]

Change https://golang.org/cl/217157 mentions this issue: internal/lsp/cache: refactor functions that return PackageHandles

[stamblerre]

At this point, the missing dependencies logic is only used in gopls to determine what types of show message warnings we should show the user. Packages with missing dependencies are only reloaded if we expect their metadata to have changed (package name or import change).