Clean up vfs.FilesystemImpl methods that operate on parent directories.

- Make FilesystemImpl methods that operate on parent directories require
  !rp.Done() (i.e. there is at least one path component to resolve) as
  precondition and postcondition (in cases where they do not finish path
  resolution due to mount boundary / absolute symlink), and require that they
  do not need to follow the last path component (the file being created /
  deleted) as a symlink. Check for these in VFS.

- Add FilesystemImpl.GetParentDentryAt(), which is required to obtain the old
  parent directory for VFS.RenameAt(). (Passing the Dentry to be renamed
  instead has the wrong semantics if the file named by the old path is a mount
  point since the Dentry will be on the wrong Mount.)

- Update memfs to implement these methods correctly (?), including RenameAt.

- Change fspath.Parse() to allow empty paths (to simplify implementation of
  AT_EMPTY_PATH).

- Change vfs.PathOperation to take a fspath.Path instead of a raw pathname;
  non-test callers will need to fspath.Parse() pathnames themselves anyway in
  order to detect absolute paths and select PathOperation.Start accordingly.

PiperOrigin-RevId: 286934941

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