go/types: remove objSet type in favor of explicit map type (cleanup)

Avoid confusion between (now gone) objSet and objset types.
Also: rename visited -> seen in initorder.go.

No functional changes.

Change-Id: Ib0aa25e006eee55a79a739194d0d26190354a9f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/198044Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/go/types/initorder.go

@@ -69,7 +69,7 @@ func (check *Checker) initOrder() {
 
 		// if n still depends on other nodes, we have a cycle
 		if n.ndeps > 0 {
-			cycle := findPath(check.objMap, n.obj, n.obj, make(objSet))
+			cycle := findPath(check.objMap, n.obj, n.obj, make(map[Object]bool))
 			// If n.obj is not part of the cycle (e.g., n.obj->b->c->d->c),
 			// cycle will be nil. Don't report anything in that case since
 			// the cycle is reported when the algorithm gets to an object
@@ -130,17 +130,17 @@ func (check *Checker) initOrder() {
 // findPath returns the (reversed) list of objects []Object{to, ... from}
 // such that there is a path of object dependencies from 'from' to 'to'.
 // If there is no such path, the result is nil.
-func findPath(objMap map[Object]*declInfo, from, to Object, visited objSet) []Object {
-	if visited[from] {
-		return nil // node already seen
+func findPath(objMap map[Object]*declInfo, from, to Object, seen map[Object]bool) []Object {
+	if seen[from] {
+		return nil
 	}
-	visited[from] = true
+	seen[from] = true
 
 	for d := range objMap[from].deps {
 		if d == to {
 			return []Object{d}
 		}
-		if P := findPath(objMap, d, to, visited); P != nil {
+		if P := findPath(objMap, d, to, seen); P != nil {
 			return append(P, d)
 		}
 	}

src/go/types/resolver.go

@@ -25,12 +25,9 @@ type declInfo struct {
 	alias bool          // type alias declaration
 
 	// The deps field tracks initialization expression dependencies.
-	deps objSet // lazily initialized
+	deps map[Object]bool // lazily initialized
 }
 
-// An objSet is simply a set of objects.
-type objSet map[Object]bool
-
 // hasInitializer reports whether the declared object has an initialization
 // expression or function body.
 func (d *declInfo) hasInitializer() bool {
@@ -41,7 +38,7 @@ func (d *declInfo) hasInitializer() bool {
 func (d *declInfo) addDep(obj Object) {
 	m := d.deps
 	if m == nil {
-		m = make(objSet)
+		m = make(map[Object]bool)
 		d.deps = m
 	}
 	m[obj] = true