Don't change position of existing autoloader when overwriting .Rprofile

In `packify()`:

- Memoize "before" and "after" part which is copied from the existing content
    - If no autoloader, "before" is the entire sequence, and "after" is empty
- Remove early return which could avoid creating `init.R` in some cases
- Always overwrite `init.R`

Fixes #164.