Module loading: load functions on init.

[jeherve]

This way we make sure everything is loaded in time, and not before the Jetpack plugin when it is installed.

I have only done a minimal amount of testing, but it seems to fix the Fatal Errors reported here:

• 无法启用插件，因为它引起了一个致命错误（fatal error）。 Automattic/jetpack#5400
• https://wordpress.org/support/topic/blanked-out-my-site/
• https://wordpress.org/support/topic/fatal-error-message-149/
• https://wordpress.org/support/topic/activating-wp-markdown-editor-results-in-white-screen/