Renamed bootstrap directory to _bootstrap to conform to default ignore
pattern of autostow.sh, and make it visually clear that this is a
special directory, not one used for the dotfiles itself.
Prepare for moving bootstrap directory to conform to new ignore pattern
of autostow.sh script. Bootstrap directory can be overwritten through
environment variable BOOTSTRAP_DIRECTORY, however for now it still
rigidly requires the existence of an install_packages.sh file and an
autostow.sh file which it wants to execute.
With bootstrap files no longer linked to config directory, we can have
an easier directory structure with less nesting for them. Install.sh
needs to use the new structure when calling its bootstrap scripts.
