go/packages: change import cycle errorkind from UnknownError to ListError

The import cycle not allowed error should be returned as a ListError not
an UnknownError.

Fixes golang/go#35964

Change-Id: Ibc575f92d926ff715c0da67a4fceda05badcc652
Reviewed-on: https://go-review.googlesource.com/c/tools/+/212138
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
Run-TryBot: Rohan Challa <rohan@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

go/packages/golist.go

@@ -811,8 +811,9 @@ func golistDriver(cfg *Config, rootsDirs func() *goInfo, words ...string) (*driv
 				msg += fmt.Sprintf(": import stack: %v", p.Error.ImportStack)
 			}
 			pkg.Errors = append(pkg.Errors, Error{
-				Pos: p.Error.Pos,
-				Msg: msg,
+				Pos:  p.Error.Pos,
+				Msg:  msg,
+				Kind: ListError,
 			})
 		}
 

internal/lsp/source/diagnostics.go

@@ -72,7 +72,7 @@ func Diagnostics(ctx context.Context, snapshot Snapshot, f File, withAnalysis bo
 	// Prepare any additional reports for the errors in this package.
 	for _, e := range pkg.GetErrors() {
 		// We only need to handle lower-level errors.
-		if !(e.Kind == UnknownError || e.Kind == ListError) {
+		if e.Kind != ListError {
 			continue
 		}
 		// If no file is associated with the error, default to the current file.
@@ -138,7 +138,7 @@ func diagnostics(ctx context.Context, snapshot Snapshot, pkg Package, reports ma
 		case TypeError:
 			set.typeErrors = append(set.typeErrors, diag)
 			diag.Source = "compiler"
-		case ListError, UnknownError:
+		case ListError:
 			set.listErrors = append(set.listErrors, diag)
 			diag.Source = "go list"
 		}