How to use get_template_part instead of include_once?

If you take a look at the Codex page for get_template_part(), you fill find the following usage:

Load a template part into a template (other than header, sidebar, footer). Makes it easy for a theme to reuse sections of code and an easy way for child themes to replace sections of their parent theme.

Includes the named template part for a theme or if a name is specified then a specialized part will be included. If the theme contains no {slug}.php file then no template will be included.

<?php get_template_part( $slug, $name ); ?>

$slug
(string) (required) The slug name for the generic template.
Default: None

$name
(string) (optional) The name of the specialized template.
Default: None

That means you can use the following:

<?php get_template_part( 'recaptchalib' ); ?>

You can use this to include every file in your theme directory.