internal/pkgbits: sync version.go with goroot

gopls/internal/golang: check dialog support directly against forms

gopls/internal/golang: refactor the file structure of dialog support

gopls/internal/server: return lazy enum or string for stub methods

gopls/internal/server: InteractiveListEnum support workspaceSymbol queries

go/types/objectpath: fix bug with methods of cyclic interfaces

go/analysis/passes/modernize: add slicesbackward analyzer

gopls/internal/golang: improve invisibility error message

gopls/internal/golang: detect package invisibility when stub methods

gopls/internal/golang: detect import cycle when stub methods

gopls/internal/protocol: preserve FormAnswers upon interactive resolution

gopls/internal/test/marker: refactor code action marker with dialog

gopls/internal/filewatcher: suppress flaky assertion on Windows

gopls/internal/server: add form for implement interface command

gopls/internal/server: implement code action implement interface X