internal/lsp: exit graceful for modfiles in semantic tokens

Change-Id: I86e6dbdd6e51af9b947c34fd04489ed34687b3c3
Reviewed-on: https://go-review.googlesource.com/c/tools/+/319849
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
diff --git a/internal/lsp/semantic.go b/internal/lsp/semantic.go
index d8feea7..c6e9190 100644
--- a/internal/lsp/semantic.go
+++ b/internal/lsp/semantic.go
@@ -77,6 +77,9 @@
 		}
 		return template.SemanticTokens(ctx, snapshot, fh.URI(), add, data)
 	}
+	if fh.Kind() != source.Go {
+		return nil, nil
+	}
 	pkg, err := snapshot.PackageForFile(ctx, fh.URI(), source.TypecheckFull, source.WidestPackage)
 	if err != nil {
 		return nil, err