fix a regression involving imported linkname funcs

In ce2c45440a, we simplified the code a bit and removed one call to
obfuscatedTypesPackage.

Unfortunately, we introduced a regression; if an exported function is
linknamed to another symbol name, and it's called from an importer
package, we would have a build failure now:

	> garble build
	[stderr]
	# test/main
	ZiOACuw7.go:1: undefined: ODC0xN52.BaDqbhkj

This is because the imported package would not hash the original name,
via its ignoreObjects logic. And, since the importer package has no
access to that knowledge, it would hash the same name, and fail to find
it in the final build.

The regression happened because we used to have a types.Scope Lookup
that saved us in this scenario. Add the test, and re-add the Lookup,
this time only for this particular scenario with function names.

Thanks to Andrew LeFevre for reporting and describing the test case.

While at it, replace more uses of "garbled" to "obfuscated".

main.go

@@ -1135,6 +1135,18 @@ func (tf *transformer) transformGo(file *ast.File) *ast.File {
 			if strings.HasPrefix(node.Name, "Test") && isTestSignature(sign) {
 				return true // don't break tests
 			}
+
+			// If this is an imported func that was linknamed to a
+			// different symbol name, the imported package did not
+			// obfuscate the original func name.
+			// Don't do it here either.
+			if parentScope != tf.pkg.Scope() {
+				if obfPkg := obfuscatedTypesPackage(path); obfPkg != nil {
+					if obfPkg.Scope().Lookup(obj.Name()) != nil {
+						return true
+					}
+				}
+			}
 		default:
 			return true // we only want to rename the above
 		}

testdata/scripts/linkname.txt

@@ -4,7 +4,7 @@ garble build
 exec ./main
 cmp stderr main.stderr
 
-! binsubstr main$exe 'garbledFunc' 'GarbledFunc'
+! binsubstr main$exe 'obfuscatedFunc' 'ObfuscatedFunc'
 
 [short] stop # no need to verify this with -short
 
@@ -24,20 +24,20 @@ import (
 	_ "strings"
 	_ "unsafe"
 
-	_ "test/main/imported"
+	"test/main/imported"
 )
 
-// A linkname to an external non-garbled func.
+// A linkname to an external non-obfuscated func.
 //go:linkname byteIndex strings.IndexByte
 func byteIndex(s string, c byte) int
 
-// A linkname to an external non-garbled non-exported func.
+// A linkname to an external non-obfuscated non-exported func.
 //go:linkname interfaceEqual os/exec.interfaceEqual
 func interfaceEqual(a, b interface{}) bool
 
-// A linkname to an external garbled func.
-//go:linkname garbledFunc test/main/imported.GarbledFuncImpl
-func garbledFunc() string
+// A linkname to an external obfuscated func.
+//go:linkname obfuscatedFunc test/main/imported.ObfuscatedFuncImpl
+func obfuscatedFunc() string
 
 // A linkname to an entirely made up name, implemented elsewhere.
 //go:linkname renamedFunc madeup.newName
@@ -46,8 +46,9 @@ func renamedFunc() string
 func main() {
 	println(byteIndex("01234", '3'))
 	println(interfaceEqual("Sephiroth", 7))
-	println(garbledFunc())
+	println(obfuscatedFunc())
 	println(renamedFunc())
+	println(imported.ByteIndex("01234", '3'))
 }
 -- imported/imported.go --
 package imported
@@ -56,16 +57,22 @@ import (
 	_ "unsafe"
 )
 
-func GarbledFuncImpl() string {
-	return "garbled func"
+func ObfuscatedFuncImpl() string {
+	return "obfuscated func"
 }
 
 //go:linkname renamedFunc madeup.newName
 func renamedFunc() string {
 	return "renamed func"
 }
+
+// A linkname to an external non-obfuscated func.
+// Different from byteIndex, as we call this from an importer package.
+//go:linkname ByteIndex strings.IndexByte
+func ByteIndex(s string, c byte) int
 -- main.stderr --
 3
 false
-garbled func
+obfuscated func
 renamed func
+3