Improve with_file API when there is no source file

[olivierlemasle]

Symbolic link files are added like this (since #168):

.with_file(
    "./test_assets/empty_file_for_symlink_create",
    FileOptions::new("/usr/bin/awesome_link")
        .mode(0o120644)
)?

However, the empty file is needed just to get its last modification timestamp, and to comply with with_file(source, options) API.

To add a directory (cf #12), it is also possible to do:

.with_file(
    "./test_assets/empty_file",
    FileOptions::new("/usr/share/awesome")
        .mode(FileMode::Dir { permissions: 0o755 }), // or .mode(0o040755)
)?

However, in both cases, it feels weird to require a file, whose content is not used.