Declared packages/* as workspaces for single-command lockstep bumping

Without the workspaces field, npm doesn't enumerate packages/cli and
packages/core when running --workspaces commands, so version bumps had
to be applied manually to each package.json. Adding the field also
properly symlinks @opkg/core and @opkg/gui under node_modules/, aligning
config with how the build script already treats them.

Author: Eric Lee