Move demandDirectoryRoot into kyaml/filesys (#4677)

* Move demandDirectoryRoot into kyaml/filesys

* Make root directory platform-agnostic

Support windows root directory. Dogfood own error package.

* Use cleaner windows support implementation

* Address feedback

* Address feedback x2

* Re-apply go.sum changes to avoid CI errors

kyaml/filesys/filesystem.go

@@ -4,7 +4,10 @@
 package filesys
 
 import (
+	"fmt"
 	"path/filepath"
+
+	"sigs.k8s.io/kustomize/kyaml/errors"
 )
 
 const (
@@ -16,40 +19,70 @@ const (
 // FileSystem groups basic os filesystem methods.
 // It's supposed be functional subset of https://golang.org/pkg/os
 type FileSystem interface {
+
 	// Create a file.
 	Create(path string) (File, error)
+
 	// MkDir makes a directory.
 	Mkdir(path string) error
+
 	// MkDirAll makes a directory path, creating intervening directories.
 	MkdirAll(path string) error
+
 	// RemoveAll removes path and any children it contains.
 	RemoveAll(path string) error
+
 	// Open opens the named file for reading.
 	Open(path string) (File, error)
+
 	// IsDir returns true if the path is a directory.
 	IsDir(path string) bool
+
 	// ReadDir returns a list of files and directories within a directory.
 	ReadDir(path string) ([]string, error)
+
 	// CleanedAbs converts the given path into a
 	// directory and a file name, where the directory
 	// is represented as a ConfirmedDir and all that implies.
 	// If the entire path is a directory, the file component
 	// is an empty string.
 	CleanedAbs(path string) (ConfirmedDir, string, error)
+
 	// Exists is true if the path exists in the file system.
 	Exists(path string) bool
+
 	// Glob returns the list of matching files,
 	// emulating https://golang.org/pkg/path/filepath/#Glob
 	Glob(pattern string) ([]string, error)
+
 	// ReadFile returns the contents of the file at the given path.
 	ReadFile(path string) ([]byte, error)
+
 	// WriteFile writes the data to a file at the given path,
 	// overwriting anything that's already there.
 	WriteFile(path string, data []byte) error
+
 	// Walk walks the file system with the given WalkFunc.
 	Walk(path string, walkFn filepath.WalkFunc) error
 }
 
+// ConfirmDir returns an error if the user-specified path is not an existing directory on fSys.
+// Otherwise, ConfirmDir returns path, which can be relative, as a ConfirmedDir and all that implies.
+func ConfirmDir(fSys FileSystem, path string) (ConfirmedDir, error) {
+	if path == "" {
+		return "", errors.Errorf("directory path cannot be empty")
+	}
+
+	d, f, err := fSys.CleanedAbs(path)
+	if err != nil {
+		return "", errors.WrapPrefixf(err, "not a valid directory")
+	}
+	if f != "" {
+		return "", errors.WrapPrefixf(errors.Errorf("file is not directory"), fmt.Sprintf("'%s'", path))
+	}
+	return d, nil
+}
+
 // FileSystemOrOnDisk satisfies the FileSystem interface by forwarding
 // all of its method calls to the given FileSystem whenever it's not nil.
 // If it's nil, the call is forwarded to the OS's underlying file system.