go/packages: avoid loading some packages in when computing overlays

In some cases, it's safe to avoid loading additional packages
when computing overlays. However it's not always safe to do so.
Avoid some unnecessary loads when it's completely safe to do so.

Updates golang/go#32538

Change-Id: Ie12204735940a540c9b3f29742f8479bcab5f077
Reviewed-on: https://go-review.googlesource.com/c/tools/+/181917
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

go/packages/golist_overlay.go

@@ -32,6 +32,13 @@ func processGolistOverlay(cfg *Config, response *responseDeduper) (modifiedPkgs,
 	var rootDirs map[string]string
 	var onceGetRootDirs sync.Once
 
+	// If no new imports are added, it is safe to avoid loading any needPkgs.
+	// Otherwise, it's hard to tell which package is actually being loaded
+	// (due to vendoring) and whether any modified package will show up
+	// in the transitive set of dependencies (because new imports are added,
+	// potentially modifying the transitive set of dependencies).
+	var overlayAddsImports bool
+
 	for opath, contents := range cfg.Overlay {
 		base := filepath.Base(opath)
 		dir := filepath.Dir(opath)
@@ -136,6 +143,7 @@ func processGolistOverlay(cfg *Config, response *responseDeduper) (modifiedPkgs,
 		for _, imp := range imports {
 			_, found := pkg.Imports[imp]
 			if !found {
+				overlayAddsImports = true
 				// TODO(matloob): Handle cases when the following block isn't correct.
 				// These include imports of test variants, imports of vendored packages, etc.
 				id, ok := havePkgs[imp]
@@ -171,10 +179,12 @@ func processGolistOverlay(cfg *Config, response *responseDeduper) (modifiedPkgs,
 		}
 	}
 
+	if overlayAddsImports {
 		needPkgs = make([]string, 0, len(needPkgsSet))
 		for pkg := range needPkgsSet {
 			needPkgs = append(needPkgs, pkg)
 		}
+	}
 	modifiedPkgs = make([]string, 0, len(modifiedPkgsSet))
 	for pkg := range modifiedPkgsSet {
 		modifiedPkgs = append(modifiedPkgs, pkg)