Get plugin dir in theme's functions.php

How do I get the plugins directory path while in my theme’s functions.php?
Suppose I have Contact form 7 installed and I want to load a file if a functions is undefined. This is what I have tried so far.

if ( ! function_exists( 'wpcf7_add_form_tag' ) ) {
    require_once PLUGINDIR . '/contact-form-7/includes/form-tags-manager.php';

But it always returns path to wp-content, which is not right because all plugins reside in web/app/plugins folder.

For one, this is a pretty standard WP question, nothing specific to Bedrock or Sage. Second, why would you need to load a function or class from a plugin in your theme? The only reason I could think of is if the plugin is not activated.

For some reasons, that function I want to use is not available in functions.php, I need that to create a shortcode to use in contact form 7 plugin.