commands: improve "Go: Add Package to Workspace" in module mode

[stamblerre]

See the discussion on microsoft/vscode-go#3195. We should improve the error reporting for this command and consider alternative ways for how this could work in module mode.

[simon0-o]

Are there any updates? This would be very convenient to view the source code of packages imported. Especially when I have to search within imported packages, now I have to launch a new window or use the integrated terminal.

[findleyr]

We discussed this in our triage meeting triage. It's a little unclear how to make this work: imported packages could be in GOCACHE.

@simon0-o how would you like this feature to behave, particularly if the package is in the module cache?

[hyangah]

@findleyr How bad can it be if we add the folders in the module cache as workspace folders?
I think that can help #2080 (ability to search code in dependency) or
https://stackoverflow.com/questions/71523201/how-to-view-external-go-module-dependencies-in-vscode.

I don't want though those folders to be diagnosed or cause to bloat gopls's memory usage footprint too much, etc
even though I appreciate analysis like reference, implemented-by, go-to-definition, documentation, ...
And, some modules in the module cache may not have go.mod file (the implicit module the go command creates to handle pre-module-time packages)

Does current gopls's data model (Session/View) handle this use case - folders in the workspace are not independent?