Use directory name as plugin dependencies slug.

[rvdsteege]

I recently noticed in my development environment that plugin dependencies are not working well if the plugin directory is not named pronamic-ideal.

In pronamic/wp-pronamic-pay-adyen#25 we started hooking into wp_plugin_dependencies_slug. We could use the same approach, but then with the directory name as slug, so any directory name is considered valid in plugin dependency checks.

Already discussed with @remcotolsma at Pronamic HQ; I'll make these changes to the code.