Remove OS-specific versions of fs.MkdirAll

Go has supported Windows paths correctly since 1.11, see
https://github.com/golang/go/issues/10900 and the commit referenced
there.

internal/fs/file.go

@@ -12,6 +12,14 @@ func Mkdir(name string, perm os.FileMode) error {
 	return os.Mkdir(fixpath(name), perm)
 }
 
+// MkdirAll creates a directory named path, along with any necessary parents,
+// and returns nil, or else returns an error. The permission bits perm are used
+// for all directories that MkdirAll creates. If path is already a directory,
+// MkdirAll does nothing and returns nil.
+func MkdirAll(path string, perm os.FileMode) error {
+	return os.MkdirAll(fixpath(path), perm)
+}
+
 // Readlink returns the destination of the named symbolic link.
 // If there is an error, it will be of type *PathError.
 func Readlink(name string) (string, error) {