Move most packages below /internal

[bep]

Hugo's package structure leaves something to be desired.

Go now has an internal package concept that we could/should start to use more to separate the public API/interest vs. the rest.

From top of the head, these would fall into the "of public interest".

• commands
• hugolib
• resources
• tpl
• deploy
• navigation
• media

And possibly some others, this needs some more thought, I thought I should raise this as an issue for a while before ... doing it.