raco pkg {install,remove}: promotion from & demotion to auto-installed

When you `raco pkg install' a package that is already present as
an auto-installed package, the installation is promoted to
an explicitly installed package.

When you `raco pkg remove --demote' a package, then it is changed
from an explicitly installed package to an auto-installed package.
Combine `--demote' with `--auto' to remove a package only
if there are no dependencies, leaving it auto-installed otherwise.

The defaults (promote in the case of `install', not demote in the case
of `remove') are different because it seems more likely that you
really mean to remove a package when using `pkg remove', while it
seems likely that you just want to start using a package that happened
to be auto-installed already for `pkg install'.

Also, make the package scope inferred for `raco pkg' commands
that take a list of package names, and fix up lock handling and
error reporting.
scribble: fix list splicing for sections
overhaul documentation for the package manager