internal/lsp: check file kind before showing warning

We had been previously popping up warning for go.mod files.

Change-Id: I6ff016e94ec46467a5c60309f825798961df66f6
Reviewed-on: https://go-review.googlesource.com/c/tools/+/254939
Trust: Rebecca Stambler <rstambler@golang.org>
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
diff --git a/internal/lsp/text_synchronization.go b/internal/lsp/text_synchronization.go
index facabbc..0a39de3 100644
--- a/internal/lsp/text_synchronization.go
+++ b/internal/lsp/text_synchronization.go
@@ -323,6 +323,13 @@
 		return
 	}
 	for _, uri := range uris {
+		fh, err := snapshot.GetFile(ctx, uri)
+		if err != nil {
+			continue
+		}
+		if fh.Kind() != source.Go {
+			continue
+		}
 		pkgs, err := snapshot.PackagesForFile(ctx, uri, source.TypecheckWorkspace)
 		if len(pkgs) > 0 || err == nil {
 			return