Plugin not working if PMPro folder name is incorrect

[dparker1005]

Describe the bug
The is_plugin_active( 'paid-memberships-pro/paid-memberships-pro.php' ) fails if the PMPro folder name is not paid-memberships-pro. This is common for release candidates.

Note that if this plugin is ever loaded before PMPro (which typically wouldn't happen since pmpro-toolkit comes before paid-memberships-pro alphabetically), this check will also fail.

Expected behavior
Use a check like defined( 'PMPRO_VERSION' ) and either:

1. Run the check in each specific place that you want to ensure that PMPro is enabled
2. Or run the check on init and, if it passes, hook all the necessary functions for the plugin to run