internal/gcimporter: fix performance regression for unified IR

flattenImports used to traverse the transitive closure of all imports by
recursively visiting all imported packages. This is a lot of redundant
work, because flattening happens for every package, the recursion is not
necessary and this change removes the recursion.

Change-Id: Id5a1b3b15ef3ce8e0fc6c945b5484b3dd06bd6b6
Reviewed-on: https://go-review.googlesource.com/c/tools/+/450755
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Florian Zenker <floriank@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

internal/gcimporter/ureader_yes.go

@@ -259,22 +259,22 @@ func (r *reader) doPkg() *types.Package {
 // packages rooted from pkgs.
 func flattenImports(pkgs []*types.Package) []*types.Package {
 	var res []*types.Package
-
-	seen := make(map[*types.Package]bool)
-	var add func(pkg *types.Package)
-	add = func(pkg *types.Package) {
-		if seen[pkg] {
-			return
-		}
-		seen[pkg] = true
-		res = append(res, pkg)
-		for _, imp := range pkg.Imports() {
-			add(imp)
-		}
-	}
-
+	seen := make(map[*types.Package]struct{})
 	for _, pkg := range pkgs {
-		add(pkg)
+		if _, ok := seen[pkg]; ok {
+			continue
+		}
+		seen[pkg] = struct{}{}
+		res = append(res, pkg)
+
+		// pkg.Imports() is already flattened.
+		for _, pkg := range pkg.Imports() {
+			if _, ok := seen[pkg]; ok {
+				continue
+			}
+			seen[pkg] = struct{}{}
+			res = append(res, pkg)
+		}
 	}
 	return res
 }