x/tools/gopls: Hover on 'package' declaration should reveal per-file language version

[adonovan]

The algorithm for deducing which version of the Go spec applies to a given Go source file, and which GODEBUG and GOEXPERIMENT flags are in force, is unfortunately rather complicated. Gopls could help by adding to the Hover result for the package declaration an explanation of which Go version is in force and why, and the values of any non-default GODEBUG and/or GOEXPERIMENT settings. (Other UI approaches are possible.)