x/tools/gopls: show progress report when gopls is downloading a module

[stamblerre]

@ardan-bkennedy suggested that we should try to use the progress reporting functionality of LSP to indicate when a module is being downloaded by go/packages. Otherwise, we just have a slow save that don't give the user any indication that gopls is communicating with the network.

This will be tricky because only go/packages knows that it's downloading a module. My current best guess at how to do this is to put something in the config's Logf function...or maybe use a trick like packagesinternal?

[ianthehat]

If the internal/telemetry experiment is a success we could instrument the go/packages library using it and then gopls could register an exporter that tracks progress and reports it easily (the same way it already intercepts log messages and sends them as "window/logMessage")
It would also replace the existing Logf hooks in a much cleaner way.

[jamall-mahmoudi-dev]

Using the Logf function
One possible way is to use the Logf function in the configuration to log messages about the start and end of the module download
Using packagesinternal:
If you want to keep the implementation details hidden, you can use an internal package (packagesinternal) to put these features in it.