obfuscate package names separately from import paths

Reflection can show package names alone via reflect.Type.String,
and I'm sure they are available in other ways.
We were obfuscating "package p" exactly like the import path "foo.com/p"
which worked OK for the most part, but not for reflection.

This is also a slight improvement to the quality of the obfuscation,
given that package names and import paths will no longer be aligned
when obfuscated.

main.go

@@ -1066,10 +1066,8 @@ func (tf *transformer) transformCompile(args []string) ([]string, error) {
 	// We don't if it's the main package, as that just uses "-p main".
 	// We only set newPkgPath if we're obfuscating the import path,
 	// to replace the original package name in the package clause below.
-	newPkgPath := ""
 	if tf.curPkg.Name != "main" && tf.curPkg.ToObfuscate {
-		newPkgPath = tf.curPkg.obfuscatedImportPath()
-		flags = flagSetValue(flags, "-p", newPkgPath)
+		flags = flagSetValue(flags, "-p", tf.curPkg.obfuscatedImportPath())
 	}
 
 	newPaths := make([]string, 0, len(files))
@@ -1090,13 +1088,7 @@ func (tf *transformer) transformCompile(args []string) ([]string, error) {
 		}
 		tf.transformDirectives(file.Comments)
 		file = tf.transformGoFile(file)
-		// newPkgPath might be the original ImportPath in some edge cases like
-		// compilerIntrinsics; we don't want to use slashes in package names.
-		// TODO: when we do away with those edge cases, only check the string is
-		// non-empty.
-		if newPkgPath != "" && newPkgPath != tf.curPkg.ImportPath {
-			file.Name.Name = newPkgPath
-		}
+		file.Name.Name = tf.curPkg.obfuscatedPackageName()
 
 		src, err := printFile(tf.curPkg, file)
 		if err != nil {
@@ -2105,9 +2097,6 @@ func (tf *transformer) transformGoFile(file *ast.File) *ast.File {
 		if err != nil {
 			panic(err) // should never happen
 		}
-		if !lpkg.ToObfuscate {
-			return true
-		}
 		if lpkg.Name != "main" {
 			newPath := lpkg.obfuscatedImportPath()
 			imp.Path.Value = strconv.Quote(newPath)

shared.go

@@ -205,6 +205,15 @@ type packageError struct {
 	Err string
 }
 
+func (p *listedPackage) obfuscatedPackageName() string {
+	// Note that package main is treated in a special way by the toolchain.
+	if p.Name == "main" || !p.ToObfuscate {
+		return p.Name
+	}
+	// The package name itself is obfuscated like any other name.
+	return hashWithPackage(p, p.Name)
+}
+
 func (p *listedPackage) obfuscatedImportPath() string {
 	// We can't obfuscate these standard library import paths,
 	// as the toolchain expects to recognize the packages by them: