support embedding via embed.FS

We already added support for "//go:embed" with string and []byte,
by not obfuscating the "embed" import path.

However, embed.FS was still failing:

	> garble build
	[stderr]
	# test/main
	:13: go:embed cannot apply to var of type embed.WtKNvwbN

The compiler detects the type by matching its name to exactly "embed.FS",
so don't obfuscate the name "FS" either.

While at it, ensure that the embed code behaves the same with "go build".

Updates #349.

main.go

@@ -1190,6 +1190,14 @@ func (tf *transformer) transformGo(file *ast.File) *ast.File {
 			return true // could be a Go plugin API
 		}
 
+		if pkg.Path() == "embed" {
+			// The Go compiler needs to detect types such as embed.FS.
+			// That will fail if we change the import path or type name.
+			// Leave it as is.
+			// Luckily, the embed package just declares the FS type.
+			return true
+		}
+
 		// We don't want to obfuscate this object.
 		if tf.ignoreObjects[obj] {
 			return true

testdata/scripts/embed.txt

@@ -5,6 +5,12 @@ garble build
 exec ./main
 cmp stdout main.stdout
 
+[short] stop # no need to verify this with -short
+
+go build
+exec ./main
+cmp stdout main.stdout
+
 -- go.mod --
 module test/main
 
@@ -14,18 +20,44 @@ go 1.16
 package main
 
 import (
-	_ "embed"
+	"embed"
 	"fmt"
+	"io/fs"
 )
 
-//go:embed test.txt
-var x string
+//go:embed embed-string.txt
+var embedStr string
+
+//go:embed embed-dir
+var embedDir embed.FS
 
 func main() {
-	fmt.Printf("%q\n", x)
+	fmt.Printf("%q\n", embedStr)
+
+	fs.WalkDir(embedDir, ".", func(path string, d fs.DirEntry, err error) error {
+		if err != nil {
+			fmt.Printf("%s: %v\n", path, err)
+			return nil
+		}
+		if !d.IsDir() {
+			body, err := fs.ReadFile(embedDir, path)
+			if err != nil {
+				fmt.Printf("%s: %v\n", path, err)
+				return nil
+			}
+			fmt.Printf("%s: %q\n", path, body)
+		}
+		return nil
+	})
 }
 
--- test.txt --
-test content
+-- embed-string.txt --
+string content
+-- embed-dir/file1.txt --
+file1 content
+-- embed-dir/file2.txt --
+file2 content
 -- main.stdout --
-"test content\n"
+"string content\n"
+embed-dir/file1.txt: "file1 content\n"
+embed-dir/file2.txt: "file2 content\n"