internal/lsp/cache: use a map to track package paths to reload

I could've sworn I'd already submitted this CL earlier. We've been
sending duplicate package paths to go/packages for no reason.

Updates golang/go#40690

Change-Id: I4c0d082a71e53df12991341b015e0ce8f504c318
Reviewed-on: https://go-review.googlesource.com/c/tools/+/248403
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>

internal/lsp/cache/snapshot.go

@@ -696,21 +696,25 @@ func (s *snapshot) reloadWorkspace(ctx context.Context) error {
 
 	// See which of the workspace packages are missing metadata.
 	s.mu.Lock()
-	var pkgPaths []interface{}
+	pkgPathSet := map[packagePath]struct{}{}
 	for id, pkgPath := range s.workspacePackages {
 		// Don't try to reload "command-line-arguments" directly.
 		if pkgPath == "command-line-arguments" {
 			continue
 		}
 		if s.metadata[id] == nil {
-			pkgPaths = append(pkgPaths, pkgPath)
+			pkgPathSet[pkgPath] = struct{}{}
 		}
 	}
 	s.mu.Unlock()
 
-	if len(pkgPaths) == 0 {
+	if len(pkgPathSet) == 0 {
 		return nil
 	}
+	var pkgPaths []interface{}
+	for pkgPath := range pkgPathSet {
+		pkgPaths = append(pkgPaths, pkgPath)
+	}
 	return s.load(ctx, pkgPaths...)
 }