Implement types for package index and packages upgrade

Related to our discussion in the mailing list, this PR includes:

- Types: `__package_{update_index,upgrade_all}` and `__package_{update_index,upgrade_all}_pacman`. If you like these types, I could implement the same for apt and yum.
- Some typos fixed in the doc
- Extra doc explaining what you told me about how to create a global "require"

Feel free to suggest any improvement :)