Add tilde expansion to file locators

[jandubois]

Required for tilde expansion in provisioning files, e.g.

❯ cat data.tmpl
base: template://docker
provision:
- mode: data
  path: /usr/local/bin/yq
  file: ~/Downloads/yq_linux_arm
  permissions: 755

(Note that this particular example will still fail because the file is too large to be included in lima.yaml as a base64-encoded string).

PR also includes a minor localpathutil.Expand() refactoring.

[AkihiroSuda]

Thanks