internal/lsp/cache: use the right snapshot in ModUpgrade

Cache computations need to use the passed in snapshot.

Fixes golang/go#41056.

Change-Id: I09501f3fc6abccedcf33482c9ee4d58146fbe716
Reviewed-on: https://go-review.googlesource.com/c/tools/+/252437
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
diff --git a/internal/lsp/cache/mod.go b/internal/lsp/cache/mod.go
index c2e80ff..e62d7fd 100644
--- a/internal/lsp/cache/mod.go
+++ b/internal/lsp/cache/mod.go
@@ -306,7 +306,7 @@
 
 		snapshot := arg.(*snapshot)
 
-		pm, err := s.ParseMod(ctx, fh)
+		pm, err := snapshot.ParseMod(ctx, fh)
 		if err != nil {
 			return &modUpgradeData{err: err}
 		}